Code/Fine-tuning.py

import re
import numpy as np
import torch
from datasets import load_dataset, Dataset
from transformers import (
    AutoModelForCausalLM,
    AutoTokenizer,
    TrainingArguments,
    Trainer,
    DataCollatorForLanguageModeling,
)
from huggingface_hub import login

##########
# CONFIG #
##########

MODEL_NAME = "Qwen/Qwen2.5-0.5B-Instruct"
DATASET = "dataset/repo"
OUTPUT_MODEL = "model/repo"

# Training hyperparams
NUM_EPOCHS = 3
PER_DEVICE_BATCH = 4
GRADIENT_ACCUMULATION = 4
LEARNING_RATE = 2e-5
WEIGHT_DECAY = 0.01
WARMUP_STEPS = 100
BF16 = True
TORCH_COMPILE = False

#########
# LOGIN #
#########

login("<YOUR_HF_TOKEN>")

##################
# LOAD TOKENIZER #
##################

tokenizer = AutoTokenizer.from_pretrained(MODEL_NAME)
tokenizer.padding_side = "right"

################
# LOAD DATASET #
################

raw_ds = load_dataset(DATASET, "default", split="train")
raw_ds = raw_ds.shuffle(seed=42)

# Apply Qwen chat template
formatted_texts = [
    tokenizer.apply_chat_template(
        conv,
        tokenize=False,
        add_generation_prompt=False
    )
    for conv in raw_ds["text"]
]

# Build simple dataset
ds = Dataset.from_dict({"text": formatted_texts})

########################
# CUSTOM DATA COLLATOR #
########################

class Qwen25DataCollator(DataCollatorForLanguageModeling):
    def __init__(self, tokenizer, mlm=False):
        super().__init__(tokenizer=tokenizer, mlm=mlm)
        # get token ids robustly (some tokenizers might return [] for encode if token missing)
        try:
            self.im_start_token = tokenizer.encode("<|im_start|>", add_special_tokens=False)[0]
        except Exception:
            self.im_start_token = None
        try:
            self.im_end_token = tokenizer.encode("<|im_end|>", add_special_tokens=False)[0]
        except Exception:
            self.im_end_token = None

        # "assistant" token sequence (may be multiple tokens)
        try:
            self.assistant_text = tokenizer.encode("assistant", add_special_tokens=False)
        except Exception:
            self.assistant_text = []

    # Provide both __call__ and torch_call for compatibility
    def __call__(self, features):
        return self.torch_call(features)

    def torch_call(self, examples):
        """
        examples: list of dicts returned by tokenization (each example contains 'input_ids', 'attention_mask', etc.)
        We'll leverage the parent to create initial batch and then mask labels for assistant responses only.
        """
        batch = super().torch_call(examples)  # returns input_ids, attention_mask, labels (for MLM)
        input_ids = batch["input_ids"]
        labels = batch["labels"]

        # If special tokens are not present, return default batch unchanged
        if self.im_start_token is None or self.im_end_token is None or len(self.assistant_text) == 0:
            return batch

        # Iterate examples in batch to mask labels: only assistant response tokens should be supervised
        for i, ids in enumerate(input_ids):
            # Find positions of <|im_start|> and <|im_end|>
            im_start_positions = torch.where(ids == self.im_start_token)[0]
            im_end_positions = torch.where(ids == self.im_end_token)[0]

            if im_start_positions.numel() == 0 or im_end_positions.numel() == 0:
                # no recognized chat markers: leave labels as-is (or continue)
                continue

            last_assistant_start = None
            # Find last im_start that is followed by "assistant"
            for start_pos in im_start_positions:
                # check if tokens following start_pos match "assistant"
                as_len = len(self.assistant_text)
                candidate_end = start_pos + 1 + as_len
                if candidate_end <= len(ids):
                    segment = ids[start_pos + 1:start_pos + 1 + as_len]
                    if torch.equal(segment, torch.tensor(self.assistant_text, device=ids.device)):
                        last_assistant_start = int(start_pos)

            if last_assistant_start is None:
                continue

            # Find first im_end after last_assistant_start
            assistant_end_positions = im_end_positions[im_end_positions > last_assistant_start]
            if assistant_end_positions.numel() == 0:
                continue

            assistant_end = int(assistant_end_positions[0])

            # Response text is between (last_assistant_start + 1 + len("assistant")) and assistant_end - 1 (inclusive),
            # but because template may include a newline or an extra token, we set response_start carefully.
            response_start = last_assistant_start + 1 + len(self.assistant_text)
            # If there's a newline token or separator, skip it if present in input_ids
            # (this is conservative: we do not assume an extra token, but we keep it if present)
            if response_start < len(ids) and ids[response_start] == tokenizer.encode("\n", add_special_tokens=False)[0]:
                response_start += 1

            # Apply masking:
            # Set everything before response_start to -100 (ignored), preserve response tokens, set rest to -100
            labels[i, :] = -100
            if response_start < len(ids):
                # labels slice up to assistant_end inclusive
                end_idx = min(assistant_end + 1, ids.shape[0])
                labels[i, response_start:end_idx] = ids[response_start:end_idx]

        # assign modified labels back
        batch["labels"] = labels
        return batch

collator = Qwen25DataCollator(tokenizer=tokenizer, mlm=False)

###############################################
# ANALYZE DATASET LENGTHS TO SET `max_length` #
###############################################

# We analyze the dataset to optimize the choice of `max_length`
print("Analyzing dataset to determine max_length (sample up to 1000)...")
assistant_lengths = []
full_lengths = []

sample_limit = min(1000, len(ds))
for example in ds["text"][:sample_limit]:
    full_tokens = tokenizer(example, truncation=False, add_special_tokens=True)
    full_lengths.append(len(full_tokens["input_ids"]))

    # extract the last assistant response via regex pattern
    pattern = r"<\|im_start\|>assistant\n(.*?)<\|im_end\|>"
    matches = re.findall(pattern, example, re.DOTALL)
    if matches:
        last_response = matches[-1]
        resp_tokens = tokenizer(last_response, truncation=False, add_special_tokens=False)
        assistant_lengths.append(len(resp_tokens["input_ids"]))

# Basic statistics (guard for empty lists)
def safe_stat(arr):
    if len(arr) == 0:
        return 0.0, 0.0, 0.0, 0.0
    return np.mean(arr), np.median(arr), np.percentile(arr, 95), np.percentile(arr, 99)

mean_ass, med_ass, p95_ass, p99_ass = safe_stat(assistant_lengths)
mean_full, _, p95_full, _ = safe_stat(full_lengths)

print(f"Assistant response mean={mean_ass:.1f}, median={med_ass:.1f}, 95%={p95_ass:.1f}, 99%={p99_ass:.1f}")
print(f"Full conversation mean={mean_full:.1f}, 95%={p95_full:.1f}")

# Round up to nearest power of two but don't exceed tokenizer.model_max_length
def next_power_of_2(x):
    if x <= 1:
        return 1
    return 2 ** int(np.ceil(np.log2(x)))

target_length = int(min(p95_full if p95_full > 0 else tokenizer.model_max_length, tokenizer.model_max_length))
MAX_LENGTH = next_power_of_2(target_length)
if MAX_LENGTH > tokenizer.model_max_length:
    MAX_LENGTH = tokenizer.model_max_length

print(f"Using MAX_LENGTH = {MAX_LENGTH}")

####################
# TOKENIZE DATASET #
####################

def tokenize_function(examples):
    return tokenizer(examples["text"], truncation=True, max_length=MAX_LENGTH, padding=False)

tokenized_ds = ds.map(tokenize_function, batched=True, remove_columns=ds.column_names)

##############
# LOAD MODEL #
##############

# Load model
model = AutoModelForCausalLM.from_pretrained(
    MODEL_NAME,
    torch_dtype=torch.bfloat16 if BF16 else None,
    device_map="auto",
    attn_implementation="flash_attention_2",
    use_cache=False,
)

try:
    from liger_kernel.transformers import apply_liger_kernel_to_qwen2
    try:
        apply_liger_kernel_to_qwen2(model)
    except TypeError:
        apply_liger_kernel_to_qwen2()
    print("Liger Kernel applied successfully for Qwen2 optimization")
except Exception:
    print("Liger Kernel not available or failed to apply; continuing without it.")

print(f"Model loaded. Parameters: {model.num_parameters() / 1e9:.3f}B")

######################
# TRAINING ARGUMENTS #
######################

training_args = TrainingArguments(
    output_dir="./qwen_rephraser_checkpoints",
    num_train_epochs=NUM_EPOCHS,
    per_device_train_batch_size=PER_DEVICE_BATCH,
    gradient_accumulation_steps=GRADIENT_ACCUMULATION,
    learning_rate=LEARNING_RATE,
    weight_decay=WEIGHT_DECAY,
    warmup_steps=WARMUP_STEPS,
    lr_scheduler_type="cosine",
    logging_steps=10,
    save_steps=500,
    save_total_limit=2,
    bf16=BF16,
    optim="adamw_torch_fused",
    gradient_checkpointing=True,
    report_to="none",
    push_to_hub=False,  # we'll push manually at the end
    hub_model_id=OUTPUT_MODEL,
    hub_private_repo=True,
    dataloader_num_workers=4,
    dataloader_pin_memory=True,
    ddp_find_unused_parameters=False,
    torch_compile=TORCH_COMPILE,
)

###########
# TRAINER #
###########

trainer = Trainer(
    model=model,
    args=training_args,
    train_dataset=tokenized_ds,
    data_collator=collator,
)

#########
# TRAIN #
#########

print("Starting training...")
trainer.train()

####################
# SAVE FINAL MODEL #
####################

print("Saving model to ./final_model ...")
model.config.use_cache = True
trainer.save_model("./final_model")
tokenizer.save_pretrained("./final_model")

##################
# PUSHING TO HUB #
##################

try:
    print(f"Pushing model and tokenizer to the hub as {OUTPUT_MODEL} (private)...")
    model.push_to_hub(OUTPUT_MODEL, private=True)
    tokenizer.push_to_hub(OUTPUT_MODEL, private=True)
    print("Push completed.")
except Exception as e:
    print("Warning: push_to_hub failed:", e)

print("Training complete!")