fix(sft): support functools.partial model.forward in chunked CE patch

tests/test_chunked_ce_partial_forward.py

@@ -0,0 +1,47 @@
+# Copyright 2020-2026 The HuggingFace Team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0
+
+import functools
+import inspect
+import types
+from unittest.mock import MagicMock
+
+import torch
+import torch.nn as nn
+
+from trl.trainer.sft_trainer import _patch_chunked_ce_lm_head
+
+
+class _TinyLM(nn.Module):
+    def __init__(self):
+        super().__init__()
+        self.config = MagicMock()
+        self.config.text_config = None
+        # top-level config attrs used when is_vlm=False
+        self.config.final_logit_softcapping = None
+        self.config.logit_scale = 1.0
+        self.config.output_router_logits = False
+        self.lm_head = nn.Linear(4, 8, bias=False)
+        self.base_model = MagicMock()
+
+    def get_output_embeddings(self):
+        return self.lm_head
+
+    def forward(self, input_ids=None, attention_mask=None, labels=None, **kwargs):
+        return MagicMock(loss=torch.tensor(0.0), logits=None)
+
+
+def test_patch_chunked_ce_accepts_partial_forward():
+    """Qwen3.5-style models expose forward as functools.partial (#6483)."""
+    model = _TinyLM()
+    real = model.forward
+    model.forward = functools.partial(real)  # no __func__
+
+    # Should not raise AttributeError: 'partial' object has no attribute '__func__'
+    _patch_chunked_ce_lm_head(model, chunk_size=2, is_vlm=False)
+
+    assert isinstance(model.forward, types.MethodType)
+    sig = inspect.signature(model.forward)
+    # Bound method signature should include original kwargs surface
+    assert "input_ids" in sig.parameters or len(sig.parameters) >= 1

trl/trainer/sft_trainer.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import contextlib
+import functools
 import inspect
 import json
 import os
@@ -373,8 +374,16 @@ def _chunked_ce_forward(
 
     # Keep the original forward signature so `generate`'s `_validate_model_kwargs` still sees the
     # model's real inputs (e.g. VLM `pixel_values`, `spatial_shapes`) and doesn't reject them. The
-    # unbound `__func__` signature makes `MethodType`'s `self`-stripping land correctly.
-    _chunked_ce_forward.__signature__ = inspect.signature(original_forward.__func__)
+    # unbound function signature makes `MethodType`'s `self`-stripping land correctly.
+    #
+    # Some multimodal models (e.g. Qwen3.5) expose `forward` as a `functools.partial` rather than a
+    # bound method, so `original_forward.__func__` is unavailable — unwrap partials / fall back to
+    # the callable itself (see #6483).
+    forward_for_sig = original_forward
+    while isinstance(forward_for_sig, functools.partial):
+        forward_for_sig = forward_for_sig.func
+    unbound = getattr(forward_for_sig, "__func__", forward_for_sig)
+    _chunked_ce_forward.__signature__ = inspect.signature(unbound)
     model.forward = types.MethodType(_chunked_ce_forward, model)