repro_bundle/extract_activations.py

"""Residual-stream activation extraction (pipeline stage M1 — the only big GPU cost).

For each (model, dataset, distribution) we run a single forward pass over the texts with
`output_hidden_states=True`, masked-mean-pool (or last-token-pool) each layer, cast to
float16, and write one cache shard. Everything downstream reads the cache.

Memory discipline (24 GB 4090):
  * model loaded in float16
  * inference under torch.no_grad()
  * per-batch hidden states pooled and moved to CPU immediately (we never keep [B,L,T,H])
  * batch_size tuned per model in config (drop for pythia-1.4b)
"""
from __future__ import annotations

import numpy as np

import cache
from config import EXTRACT, MODELS, ExtractConfig


def _pool(hidden_stack, attention_mask, pooling: str):
    """hidden_stack: [B, L+1, T, H] ; attention_mask: [B, T] -> [B, L+1, H]."""
    import torch
    if pooling == "mean":
        mask = attention_mask[:, None, :, None]                 # [B,1,T,1]
        summed = (hidden_stack * mask).sum(dim=2)               # [B,L+1,H]
        counts = mask.sum(dim=2).clamp(min=1)                   # [B,1,H]->broadcast
        return summed / counts
    if pooling == "last":
        # index of last non-pad token per example
        lengths = attention_mask.sum(dim=1).long() - 1          # [B]
        idx = lengths[:, None, None, None].expand(-1, hidden_stack.size(1), 1, hidden_stack.size(3))
        return torch.gather(hidden_stack, 2, idx).squeeze(2)    # [B,L+1,H]
    raise ValueError(f"unknown pooling {pooling}")


def extract_texts(model_key: str, texts: list[str], cfg: ExtractConfig) -> np.ndarray:
    """Return pooled activations [N, L+1, H] (float16) for the given texts."""
    import torch
    from transformers import AutoModel, AutoTokenizer

    spec = MODELS[model_key]
    tok = AutoTokenizer.from_pretrained(spec.hf_name)
    if tok.pad_token is None:
        tok.pad_token = tok.eos_token
    dtype = getattr(torch, cfg.dtype)
    device = cfg.device if torch.cuda.is_available() else "cpu"
    model = AutoModel.from_pretrained(
        spec.hf_name, torch_dtype=dtype, output_hidden_states=True
    ).to(device).eval()

    chunks = []
    with torch.no_grad():
        for i in range(0, len(texts), cfg.batch_size):
            batch = texts[i:i + cfg.batch_size]
            enc = tok(batch, return_tensors="pt", padding=True, truncation=True,
                      max_length=cfg.max_length).to(device)
            out = model(**enc)
            hs = torch.stack(out.hidden_states, dim=1)          # [B, L+1, T, H]
            pooled = _pool(hs, enc.attention_mask, cfg.pooling) # [B, L+1, H]
            chunks.append(pooled.to(torch.float16).cpu().numpy())
            del out, hs, pooled, enc

    del model
    if torch.cuda.is_available():
        torch.cuda.empty_cache()
    return np.concatenate(chunks, axis=0)


def extract_shard(model_key: str, dataset_key: str, distribution: str, record: dict,
                  cfg: ExtractConfig = EXTRACT, overwrite: bool = False):
    """Extract + cache one shard. `record` = {texts, labels, ids}."""
    if len(record["texts"]) == 0:
        return None   # empty record (e.g. paraphrase fully filtered out) -> skip, don't crash
    if cache.exists(model_key, dataset_key, distribution) and not overwrite:
        return cache.shard_dir(model_key, dataset_key, distribution)
    acts = extract_texts(model_key, record["texts"], cfg)
    return cache.save_shard(model_key, dataset_key, distribution,
                            acts, record["labels"], record["ids"], cfg.pooling)