Fix Issue #40913: Respect user-provided chat_template parameter in processor creation

[qizwiz]

This PR fixes a bug where user-provided chat_template parameters were being overwritten by model defaults when creating a processor. The fix ensures that user-provided values take precedence over model defaults.

Fixes #40913

Problem

When creating a processor with a user-provided chat_template parameter, the value was being overwritten by the model's default chat template. This prevented users from customizing the chat template as intended.

Reproduction:

from transformers import AutoProcessor
processor = AutoProcessor.from_pretrained("Qwen/Qwen2.5-VL-3B-Instruct", chat_template="test")
print(processor.chat_template)  # Should print "test" but prints the default template

Root Cause

In src/transformers/processing_utils.py line 1086, the code was unconditionally setting:

if chat_templates:
    kwargs["chat_template"] = chat_templates

This overwrote any user-provided chat_template parameter.

Solution

The fix modifies the condition to only set the chat template from model files if no user-provided value exists:

if chat_templates and "chat_template" not in kwargs:
    kwargs["chat_template"] = chat_templates

This ensures that:

1. User-provided chat_template values take precedence
2. Model defaults are still used when no user value is provided
3. Backward compatibility is maintained

Testing

Added a test case that verifies:

1. User-provided chat_template values are preserved
2. Model defaults are used when no user value is provided

Impact

This is a backward-compatible bug fix that allows users to override chat templates as expected when loading processors.

[zucchini-nlp]

Can you add a proper test please?

[zucchini-nlp]

?

[zucchini-nlp]

can do kwargs.set_default("chat_template", chat_templates)