transformer-1b-chat/training_code/model/sft_data.py

"""
SFT data pipeline: loads UltraChat 200K and formats into chat template.

Chat template:
  <|user|>
  What is gravity?
  <|end|>
  <|assistant|>
  Gravity is a fundamental force...
  <|end|>

Labels are shifted left by 1 (standard causal LM), with user turns masked.
"""

import torch
from torch.utils.data import Dataset, DataLoader
from datasets import load_dataset


CHAT_TEMPLATE = {
    "user_start": "<|user|>\n",
    "assistant_start": "<|assistant|>\n",
    "turn_end": "\n<|end|>\n",
}


def format_conversation(messages):
    """Convert a list of {role, content} messages into our chat template string."""
    text = ""
    for msg in messages:
        role = msg["role"]
        content = msg["content"].strip()
        if role == "user":
            text += CHAT_TEMPLATE["user_start"] + content + CHAT_TEMPLATE["turn_end"]
        elif role == "assistant":
            text += CHAT_TEMPLATE["assistant_start"] + content + CHAT_TEMPLATE["turn_end"]
    return text


class SFTDataset(Dataset):
    """
    Loads UltraChat 200K conversations, tokenizes them, builds shifted labels
    with user turns masked so the model only learns to generate assistant responses.
    """

    def __init__(self, tokenizer, max_seq_len=2048, split="train_sft", cache_dir=None, max_samples=None):
        self.tokenizer = tokenizer
        self.max_seq_len = max_seq_len

        special_tokens = ["<|user|>", "<|assistant|>", "<|end|>"]
        vocab = tokenizer.get_vocab()
        new_tokens = [t for t in special_tokens if t not in vocab]
        if new_tokens:
            tokenizer.add_tokens(new_tokens, special_tokens=True)

        self.assistant_token_id = tokenizer.encode("<|assistant|>", add_special_tokens=False)[0]
        self.end_token_id = tokenizer.encode("<|end|>", add_special_tokens=False)[0]
        self.user_token_id = tokenizer.encode("<|user|>", add_special_tokens=False)[0]

        print(f"[SFT Data] Loading UltraChat 200K ({split})...")
        ds = load_dataset("HuggingFaceH4/ultrachat_200k", split=split, cache_dir=cache_dir)
        if max_samples:
            ds = ds.select(range(min(max_samples, len(ds))))
        print(f"[SFT Data] {len(ds)} conversations loaded")

        self.examples = []
        skipped = 0
        for i, row in enumerate(ds):
            messages = row["messages"]
            if len(messages) < 2:
                skipped += 1
                continue

            text = format_conversation(messages)
            all_ids = tokenizer.encode(text, add_special_tokens=False)

            # Need at least max_seq_len+1 for shift, but truncate if longer
            if len(all_ids) > max_seq_len + 1:
                all_ids = all_ids[:max_seq_len + 1]

            if len(all_ids) < 10:
                skipped += 1
                continue

            # Shifted: input = all_ids[:-1], target = all_ids[1:]
            input_ids = all_ids[:-1]
            target_ids = all_ids[1:]

            # Build mask: -100 for user turns, real token id for assistant turns
            labels = self._build_shifted_labels(input_ids, target_ids)
            self.examples.append((input_ids, labels))

            if (i + 1) % 50000 == 0:
                print(f"  Processed {i+1} conversations...")

        print(f"[SFT Data] {len(self.examples)} examples ready, {skipped} skipped")

    def _build_shifted_labels(self, input_ids, target_ids):
        """
        Walk through the token sequence and track whether we're in a user turn
        or assistant turn. Only keep labels for assistant response content.

        Masking strategy (applied to the SHIFTED target):
        - Everything before and including <|assistant|>\\n: masked
        - Assistant response content and <|end|>: TRAIN
        - <|user|> and user content until next <|assistant|>: masked
        """
        labels = [-100] * len(target_ids)
        in_assistant = False

        for i, tid in enumerate(input_ids):
            if tid == self.assistant_token_id:
                # Next token after <|assistant|> is \n, then content starts
                in_assistant = True
                continue

            if tid == self.user_token_id:
                in_assistant = False
                continue

            if in_assistant:
                labels[i] = target_ids[i]

            # When we hit <|end|> in assistant mode, include it then switch off
            if tid == self.end_token_id and in_assistant:
                in_assistant = False

        return labels

    def __len__(self):
        return len(self.examples)

    def __getitem__(self, idx):
        input_ids, labels = self.examples[idx]
        return torch.tensor(input_ids, dtype=torch.long), torch.tensor(labels, dtype=torch.long)


def sft_collate_fn(batch, pad_id=0):
    """Pad sequences to the same length within a batch."""
    input_ids_list, labels_list = zip(*batch)
    max_len = max(ids.size(0) for ids in input_ids_list)

    padded_inputs = []
    padded_labels = []
    for ids, lbl in zip(input_ids_list, labels_list):
        pad_len = max_len - ids.size(0)
        padded_inputs.append(torch.cat([ids, torch.full((pad_len,), pad_id, dtype=torch.long)]))
        padded_labels.append(torch.cat([lbl, torch.full((pad_len,), -100, dtype=torch.long)]))

    return torch.stack(padded_inputs), torch.stack(padded_labels)


def create_sft_dataloader(tokenizer, batch_size=4, max_seq_len=2048,
                          cache_dir=None, max_samples=None, num_workers=4):
    dataset = SFTDataset(
        tokenizer=tokenizer,
        max_seq_len=max_seq_len,
        split="train_sft",
        cache_dir=cache_dir,
        max_samples=max_samples,
    )
    return DataLoader(
        dataset,
        batch_size=batch_size,
        shuffle=True,
        num_workers=num_workers,
        pin_memory=True,
        collate_fn=lambda b: sft_collate_fn(b, pad_id=tokenizer.pad_token_id),
    ), dataset