Fix custom processors that use deleted behaviour for Transformers v5 (#35107)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

vllm/transformers_utils/processor.py

@@ -44,7 +44,39 @@ def _transformers_v4_compatibility_import():
         processing_utils.ChatTemplateLoadKwargs = new_import
 
 
+def _transformers_v4_compatibility_init() -> Any:
+    """Some remote code processors may define `optional_attributes` in their
+    `ProcessorMixin` subclass, and then pass these arbitrary attributes directly to
+    `ProcessorMixin.__init__`, which is no longer allowed in Transformers v5. For
+    backward compatibility, we intercept these optional attributes and set them on the
+    processor instance before calling the original `ProcessorMixin.__init__`.
+
+    This can be removed if `Molmo2ForConditionalGeneration` is upstreamed to
+    Transformers."""
+    # Transformers v4
+    if hasattr(ProcessorMixin, "optional_attributes"):
+        return
+    # Transformers v5
+    if hasattr(ProcessorMixin.__init__, "_vllm_patched"):
+        return
+
+    original_init = ProcessorMixin.__init__
+
+    def __init__(self, *args, **kwargs):
+        for optional_attribute in getattr(self, "optional_attributes", []):
+            if optional_attribute in kwargs:
+                setattr(self, optional_attribute, kwargs.pop(optional_attribute))
+
+        original_init(self, *args, **kwargs)
+
+    # Only patch if ProcessorMixin is not mocked (for docs builds)
+    if not hasattr(ProcessorMixin, "_mock_name"):
+        __init__._vllm_patched = True  # type: ignore[attr-defined]
+        ProcessorMixin.__init__ = __init__
+
+
 _transformers_v4_compatibility_import()
+_transformers_v4_compatibility_init()
 
 _P = TypeVar("_P", bound=ProcessorMixin, default=ProcessorMixin)
 _V = TypeVar("_V", bound=BaseVideoProcessor, default=BaseVideoProcessor)