src/kfac_collector.py

"""
K-FAC Statistics Collector

Collects activation covariance (A) and gradient covariance (G) matrices
for MLP layers to approximate the Fisher Information Matrix.

Based on: "From Memorization to Reasoning in the Spectrum of Loss Curvature"
"""

import torch
import torch.nn as nn
from torch import Tensor
from typing import Optional, Callable
from dataclasses import dataclass, field
from tqdm import tqdm


@dataclass
class LayerStatistics:
    """K-FAC statistics for a single layer."""
    
    # Covariance matrices
    A: Optional[Tensor] = None  # Activation covariance (d_in x d_in)
    G: Optional[Tensor] = None  # Gradient covariance (d_out x d_out)
    
    # Running sums for incremental computation
    A_sum: Optional[Tensor] = None
    G_sum: Optional[Tensor] = None
    
    # Counts
    n_samples_A: int = 0
    n_samples_G: int = 0
    
    def finalize(self) -> None:
        """Convert running sums to means."""
        if self.A_sum is not None and self.n_samples_A > 0:
            self.A = self.A_sum / self.n_samples_A
            self.A_sum = None
        if self.G_sum is not None and self.n_samples_G > 0:
            self.G = self.G_sum / self.n_samples_G
            self.G_sum = None


@dataclass
class KFACConfig:
    """Configuration for K-FAC collection."""
    
    # Target layers (layer indices)
    target_layers: list[int] = field(default_factory=lambda: [11, 12, 13])
    
    # Target projections within MLP
    target_projections: list[str] = field(default_factory=lambda: ["gate_proj", "up_proj"])
    
    # Sequence length for batching
    seq_length: int = 512
    
    # Whether to exclude last position (for causal LM)
    exclude_last_position: bool = True
    
    # Use sampled labels instead of ground truth for proper FIM
    sample_labels: bool = True
    
    # Device
    device: str = "cuda"


class KFACCollector:
    """
    Collects K-FAC statistics (activation and gradient covariances) for MLP layers.
    
    The K-FAC approximation factorizes the Fisher Information Matrix as:
        F_W ≈ G ⊗ A = E[gg^T] ⊗ E[aa^T]
    
    where:
        - A is the covariance of activations going into the layer
        - G is the covariance of gradients on the layer's output
    
    Usage:
        collector = KFACCollector(model, config)
        collector.register_hooks()
        
        for batch in dataloader:
            collector.collect_batch(batch, tokenizer)
        
        collector.finalize()
        collector.save("kfac_stats.pt")
    """
    
    def __init__(
        self,
        model: nn.Module,
        config: Optional[KFACConfig] = None,
    ):
        self.model = model
        self.config = config or KFACConfig()
        
        # Storage for statistics
        self.stats: dict[str, LayerStatistics] = {}
        
        # Hooks
        self._forward_hooks: list = []
        self._backward_hooks: list = []
        
        # Buffers for current batch
        self._activation_buffer: dict[str, Tensor] = {}
        self._gradient_buffer: dict[str, Tensor] = {}
        
        # Track registration state
        self._hooks_registered = False
        
    def _get_layer_name(self, layer_idx: int, proj_name: str) -> str:
        """Generate a unique name for a layer/projection combination."""
        return f"layer_{layer_idx}.{proj_name}"
    
    def _get_target_modules(self) -> dict[str, nn.Linear]:
        """Find all target modules in the model."""
        targets = {}
        
        # Handle different model architectures
        # OLMo-2 / LLaMA style: model.layers[i].mlp.{gate_proj, up_proj, down_proj}
        
        layers = None
        if hasattr(self.model, "model") and hasattr(self.model.model, "layers"):
            # HF style (e.g., OLMoForCausalLM)
            layers = self.model.model.layers
        elif hasattr(self.model, "transformer") and hasattr(self.model.transformer, "blocks"):
            # GPT style
            layers = self.model.transformer.blocks
        elif hasattr(self.model, "layers"):
            # Direct access
            layers = self.model.layers
        
        if layers is None:
            raise ValueError(
                "Could not find transformer layers. "
                "Model architecture not recognized. "
                f"Model type: {type(self.model)}"
            )
        
        for layer_idx in self.config.target_layers:
            if layer_idx >= len(layers):
                print(f"Warning: Layer {layer_idx} does not exist (model has {len(layers)} layers)")
                continue
                
            layer = layers[layer_idx]
            
            # Find MLP submodule
            mlp = None
            if hasattr(layer, "mlp"):
                mlp = layer.mlp
            elif hasattr(layer, "feed_forward"):
                mlp = layer.feed_forward
            elif hasattr(layer, "ff"):
                mlp = layer.ff
            
            if mlp is None:
                print(f"Warning: Could not find MLP in layer {layer_idx}")
                continue
            
            for proj_name in self.config.target_projections:
                if hasattr(mlp, proj_name):
                    proj = getattr(mlp, proj_name)
                    if isinstance(proj, nn.Linear):
                        name = self._get_layer_name(layer_idx, proj_name)
                        targets[name] = proj
                        self.stats[name] = LayerStatistics()
                else:
                    print(f"Warning: {proj_name} not found in layer {layer_idx}")
        
        return targets
    
    def register_hooks(self) -> None:
        """Register forward and backward hooks on target modules."""
        if self._hooks_registered:
            print("Hooks already registered")
            return
        
        targets = self._get_target_modules()
        
        if not targets:
            raise ValueError("No target modules found to hook")
        
        print(f"Registering hooks on {len(targets)} modules:")
        for name in targets:
            print(f"  - {name}")
        
        for name, module in targets.items():
            # Forward hook: capture input activations
            def make_forward_hook(layer_name: str):
                def hook(module: nn.Module, input: tuple, output: Tensor) -> None:
                    # input is a tuple, first element is the activation tensor
                    x = input[0]
                    if x.requires_grad:
                        # Store for backward pass
                        self._activation_buffer[layer_name] = x.detach()
                return hook
            
            # Backward hook: capture output gradients
            def make_backward_hook(layer_name: str):
                def hook(module: nn.Module, grad_input: tuple, grad_output: tuple) -> None:
                    # grad_output is tuple, first element is gradient w.r.t. output
                    g = grad_output[0]
                    if g is not None:
                        self._gradient_buffer[layer_name] = g.detach()
                return hook
            
            fh = module.register_forward_hook(make_forward_hook(name))
            bh = module.register_full_backward_hook(make_backward_hook(name))
            
            self._forward_hooks.append(fh)
            self._backward_hooks.append(bh)
        
        self._hooks_registered = True
    
    def remove_hooks(self) -> None:
        """Remove all registered hooks."""
        for hook in self._forward_hooks:
            hook.remove()
        for hook in self._backward_hooks:
            hook.remove()
        
        self._forward_hooks = []
        self._backward_hooks = []
        self._hooks_registered = False
    
    def _update_statistics(self) -> None:
        """Update running statistics from current buffers."""
        for name in self.stats:
            if name in self._activation_buffer:
                x = self._activation_buffer[name]
                # x shape: (batch, seq_len, d_in)
                
                # Optionally exclude last position
                if self.config.exclude_last_position and x.shape[1] > 1:
                    x = x[:, :-1, :]
                
                # Flatten batch and sequence dimensions
                x_flat = x.reshape(-1, x.shape[-1])  # (batch * seq, d_in)
                n_positions = x_flat.shape[0]
                
                # Compute A contribution: x^T @ x
                A_batch = x_flat.T @ x_flat  # (d_in, d_in)
                
                # Update running sum
                if self.stats[name].A_sum is None:
                    self.stats[name].A_sum = A_batch
                else:
                    self.stats[name].A_sum = self.stats[name].A_sum + A_batch
                self.stats[name].n_samples_A += n_positions
            
            if name in self._gradient_buffer:
                g = self._gradient_buffer[name]
                # g shape: (batch, seq_len, d_out)
                
                # Optionally exclude last position
                if self.config.exclude_last_position and g.shape[1] > 1:
                    g = g[:, :-1, :]
                
                # Flatten batch and sequence dimensions
                g_flat = g.reshape(-1, g.shape[-1])  # (batch * seq, d_out)
                n_positions = g_flat.shape[0]
                
                # Compute G contribution: g^T @ g
                G_batch = g_flat.T @ g_flat  # (d_out, d_out)
                
                # Update running sum
                if self.stats[name].G_sum is None:
                    self.stats[name].G_sum = G_batch
                else:
                    self.stats[name].G_sum = self.stats[name].G_sum + G_batch
                self.stats[name].n_samples_G += n_positions
        
        # Clear buffers
        self._activation_buffer.clear()
        self._gradient_buffer.clear()
    
    @torch.no_grad()
    def _sample_labels(self, logits: Tensor) -> Tensor:
        """
        Sample labels from model's predicted distribution.
        
        For proper FIM computation, we sample ŷ ~ p(y|x) rather than
        using ground truth labels.
        """
        # logits shape: (batch, seq_len, vocab_size)
        probs = torch.softmax(logits, dim=-1)
        # Sample from categorical distribution
        sampled = torch.multinomial(
            probs.view(-1, probs.shape[-1]), 
            num_samples=1
        ).view(probs.shape[:-1])
        return sampled
    
    def collect_batch(
        self,
        input_ids: Tensor,
        attention_mask: Optional[Tensor] = None,
        labels: Optional[Tensor] = None,
    ) -> float:
        """
        Collect K-FAC statistics from a single batch.
        
        Args:
            input_ids: Token IDs (batch, seq_len)
            attention_mask: Attention mask (batch, seq_len)
            labels: Ground truth labels (optional, will be sampled if not provided
                    or if config.sample_labels is True)
        
        Returns:
            Loss value for this batch
        """
        self.model.train()  # Need gradients
        
        # Move to device
        input_ids = input_ids.to(self.config.device)
        if attention_mask is not None:
            attention_mask = attention_mask.to(self.config.device)
        
        # Forward pass
        with torch.enable_grad():
            outputs = self.model(
                input_ids=input_ids,
                attention_mask=attention_mask,
                use_cache=False,
            )
            logits = outputs.logits
            
            # Get labels for loss computation
            if self.config.sample_labels or labels is None:
                # Sample from model's distribution (proper FIM)
                sampled_labels = self._sample_labels(logits)
                # Shift for causal LM
                shift_labels = sampled_labels[:, 1:].contiguous()
                shift_logits = logits[:, :-1, :].contiguous()
            else:
                # Use provided labels
                shift_labels = labels[:, 1:].contiguous().to(self.config.device)
                shift_logits = logits[:, :-1, :].contiguous()
            
            # Compute loss
            loss_fn = nn.CrossEntropyLoss()
            loss = loss_fn(
                shift_logits.view(-1, shift_logits.shape[-1]),
                shift_labels.view(-1)
            )
            
            # Backward pass to populate gradient buffers
            loss.backward()
        
        # Update statistics from buffers
        self._update_statistics()
        
        # Zero gradients for next batch
        self.model.zero_grad()
        
        return loss.item()
    
    def collect_from_dataloader(
        self,
        dataloader,
        max_tokens: int = 20_000_000,
        progress_bar: bool = True,
    ) -> dict:
        """
        Collect K-FAC statistics from a dataloader.
        
        Args:
            dataloader: PyTorch DataLoader yielding batches with input_ids
            max_tokens: Maximum number of tokens to process
            progress_bar: Whether to show progress bar
        
        Returns:
            Dictionary with collection statistics
        """
        if not self._hooks_registered:
            self.register_hooks()
        
        total_tokens = 0
        total_loss = 0.0
        n_batches = 0
        
        iterator = tqdm(dataloader, desc="Collecting K-FAC stats") if progress_bar else dataloader
        
        for batch in iterator:
            if isinstance(batch, dict):
                input_ids = batch["input_ids"]
                attention_mask = batch.get("attention_mask")
            else:
                input_ids = batch[0]
                attention_mask = batch[1] if len(batch) > 1 else None
            
            batch_tokens = input_ids.numel()
            
            loss = self.collect_batch(input_ids, attention_mask)
            
            total_tokens += batch_tokens
            total_loss += loss
            n_batches += 1
            
            if progress_bar:
                iterator.set_postfix({
                    "tokens": f"{total_tokens/1e6:.1f}M",
                    "loss": f"{loss:.3f}"
                })
            
            if total_tokens >= max_tokens:
                break
        
        return {
            "total_tokens": total_tokens,
            "n_batches": n_batches,
            "avg_loss": total_loss / max(n_batches, 1),
        }
    
    def finalize(self) -> None:
        """Finalize statistics by converting sums to means."""
        for name, stat in self.stats.items():
            stat.finalize()
            print(f"Finalized {name}: A={stat.A.shape if stat.A is not None else None}, "
                  f"G={stat.G.shape if stat.G is not None else None}")
    
    def save(self, path: str) -> None:
        """Save K-FAC statistics to file."""
        save_dict = {
            "config": {
                "target_layers": self.config.target_layers,
                "target_projections": self.config.target_projections,
                "seq_length": self.config.seq_length,
            },
            "statistics": {}
        }
        
        for name, stat in self.stats.items():
            save_dict["statistics"][name] = {
                "A": stat.A.cpu() if stat.A is not None else None,
                "G": stat.G.cpu() if stat.G is not None else None,
                "n_samples_A": stat.n_samples_A,
                "n_samples_G": stat.n_samples_G,
            }
        
        torch.save(save_dict, path)
        print(f"Saved K-FAC statistics to {path}")
    
    @classmethod
    def load(cls, path: str, model: nn.Module) -> "KFACCollector":
        """Load K-FAC statistics from file."""
        data = torch.load(path, map_location="cpu")
        
        config = KFACConfig(
            target_layers=data["config"]["target_layers"],
            target_projections=data["config"]["target_projections"],
            seq_length=data["config"]["seq_length"],
        )
        
        collector = cls(model, config)
        
        for name, stat_data in data["statistics"].items():
            collector.stats[name] = LayerStatistics(
                A=stat_data["A"],
                G=stat_data["G"],
                n_samples_A=stat_data["n_samples_A"],
                n_samples_G=stat_data["n_samples_G"],
            )
        
        print(f"Loaded K-FAC statistics from {path}")
        return collector
    
    def get_statistics(self) -> dict[str, tuple[Tensor, Tensor]]:
        """
        Get computed A and G matrices for all layers.
        
        Returns:
            Dictionary mapping layer names to (A, G) tuples
        """
        result = {}
        for name, stat in self.stats.items():
            if stat.A is not None and stat.G is not None:
                result[name] = (stat.A, stat.G)
        return result


def create_dataloader(
    dataset_name: str = "allenai/c4",
    dataset_config: str = "en",  # English subset
    tokenizer = None,
    batch_size: int = 4,
    seq_length: int = 512,
    max_samples: Optional[int] = None,
    streaming: bool = True,
    shuffle_buffer: int = 10000,
    seed: int = 42,
):
    """
    Create a DataLoader for K-FAC collection.
    
    Args:
        dataset_name: HuggingFace dataset name
        dataset_config: Dataset configuration/subset name
        tokenizer: Tokenizer for the model
        batch_size: Batch size
        seq_length: Sequence length for tokenization
        max_samples: Maximum number of samples to load
        streaming: Whether to use streaming mode
        shuffle_buffer: Buffer size for streaming shuffle
        seed: Random seed
    
    Returns:
        PyTorch DataLoader
    """
    from datasets import load_dataset
    from torch.utils.data import DataLoader, IterableDataset
    
    # Load dataset
    if dataset_config:
        ds = load_dataset(dataset_name, name=dataset_config, split="train", streaming=streaming)
    else:
        ds = load_dataset(dataset_name, split="train", streaming=streaming)
    
    if streaming:
        ds = ds.shuffle(buffer_size=shuffle_buffer, seed=seed)
    
    # Tokenization function
    def tokenize_fn(examples):
        # Handle different column names
        text_column = "text" if "text" in examples else list(examples.keys())[0]
        texts = examples[text_column]
        
        tokenized = tokenizer(
            texts,
            truncation=True,
            max_length=seq_length,
            padding="max_length",
            return_tensors="pt",
        )
        return tokenized
    
    # Create streaming dataset wrapper
    class TokenizedIterableDataset(IterableDataset):
        def __init__(self, dataset, tokenizer, seq_length, max_samples):
            self.dataset = dataset
            self.tokenizer = tokenizer
            self.seq_length = seq_length
            self.max_samples = max_samples
        
        def __iter__(self):
            count = 0
            for example in self.dataset:
                if self.max_samples and count >= self.max_samples:
                    break
                
                # Get text
                text = example.get("text", list(example.values())[0])
                if not text:
                    continue
                
                # Tokenize
                tokens = self.tokenizer(
                    text,
                    truncation=True,
                    max_length=self.seq_length,
                    padding="max_length",
                    return_tensors="pt",
                )
                
                yield {
                    "input_ids": tokens["input_ids"].squeeze(0),
                    "attention_mask": tokens["attention_mask"].squeeze(0),
                }
                count += 1
    
    torch_dataset = TokenizedIterableDataset(ds, tokenizer, seq_length, max_samples)
    
    dataloader = DataLoader(
        torch_dataset,
        batch_size=batch_size,
        num_workers=0,  # Streaming doesn't work well with multiple workers
    )
    
    return dataloader