[algo] add ASFT (#10174)

src/llamafactory/train/trainer_utils.py

@@ -23,6 +23,7 @@ from collections.abc import Callable, Mapping
 from typing import TYPE_CHECKING, Any, Optional, Union
 
 import torch
+import torch.nn.functional as F
 from transformers import Trainer
 from transformers.integrations import is_deepspeed_zero3_enabled
 from transformers.modeling_utils import is_fsdp_enabled
@@ -681,6 +682,88 @@ def _dft_cross_entropy(
     return loss
 
 
+def asft_loss_func(
+    outputs,
+    labels: torch.Tensor,
+    ref_logits: torch.Tensor,
+    asft_alpha: float = 0.1,
+    ignore_index: int = -100,
+) -> torch.Tensor:
+    logits = outputs.get("logits")
+    if logits is None:
+        return outputs.get("loss", torch.tensor(0.0))
+
+    logits = logits.float()
+
+    # shift for causal LM
+    shift_logits = logits[..., :-1, :].contiguous()
+    shift_labels = labels[..., 1:].contiguous()
+    shift_ref_logits = ref_logits[..., :-1, :].contiguous()
+
+    vocab_size = shift_logits.size(-1)
+
+    # flatten
+    shift_logits = shift_logits.view(-1, vocab_size)
+    shift_ref_logits = shift_ref_logits.view(-1, vocab_size)
+    shift_labels = shift_labels.view(-1).to(shift_logits.device)
+
+    return _asft_cross_entropy(
+        policy_logits=shift_logits,
+        policy_labels=shift_labels,
+        ref_logits=shift_ref_logits,
+        asft_alpha=asft_alpha,
+        ignore_index=ignore_index,
+    )
+
+
+def _asft_cross_entropy(
+    policy_logits: torch.Tensor,
+    policy_labels: torch.Tensor,
+    ref_logits: torch.Tensor,
+    asft_alpha: float = 0.1,
+    ignore_index: int = -100,
+) -> torch.Tensor:
+    dft_loss = _dft_cross_entropy(
+        policy_logits,
+        policy_labels,
+        ignore_index=ignore_index,
+    )
+
+    kl_loss = _kl_divergence(
+        policy_logits,
+        ref_logits,
+        policy_labels,
+        ignore_index=ignore_index,
+    )
+
+    return dft_loss + asft_alpha * kl_loss
+
+
+def _kl_divergence(
+    policy_logits: torch.Tensor,
+    ref_logits: torch.Tensor,
+    labels: torch.Tensor,
+    ignore_index: int = -100,
+) -> torch.Tensor:
+    # log p(y|x)
+    log_p = F.log_softmax(policy_logits, dim=-1)
+
+    # q(y|x)
+    q = F.softmax(ref_logits, dim=-1)
+
+    # token-wise KL
+    kl = F.kl_div(
+        log_p,
+        q,
+        reduction="none",
+    ).sum(dim=-1)  # [N]
+
+    # mask padding tokens
+    mask = (labels != ignore_index).float()
+
+    return (kl * mask).sum() / mask.sum()
+
+
 def eaft_loss_func(
     outputs: "torch.Tensor",
     labels: "torch.Tensor",