Fix assistant_masks for multimodal inputs in apply_chat_template

[umbilnm]

What does this PR do?

Fixes #44521

apply_chat_template with return_assistant_tokens_mask=True returns all-zero masks when multimodal inputs (images/videos) are present.

Root cause

generation_indices (character-level positions of assistant responses) are computed from the original prompt text rendered by Jinja, which contains a single placeholder token per image (e.g. one <|image_pad|>). However, the processor's __call__ expands each placeholder into N copies (based on image resolution), so offset_mapping returned by the tokenizer corresponds to the expanded text. The bisect_left lookup then fails to find the assistant span, and the mask stays all zeros.

Fix

When multimodal inputs are present:

1. Tokenize the original (unexpanded) prompt separately to get offset_mapping aligned with generation_indices
2. Build the assistant mask on the original tokenization (where bisect_left works correctly)
3. Map the mask onto the expanded input_ids via two-pointer alignment — matching tokens get their mask value, extra expansion tokens get 0

This approach is generic and works for any multimodal processor that expands placeholder tokens, without requiring model-specific logic.

When no multimodal inputs are present, the original code path is used unchanged.

Tests

Added test_apply_chat_template_assistant_mask_with_image in test_processing_common.py. Verified on Qwen2.5-VL and Qwen3-VL:

• Without fix: FAIL (mask is all zeros)
• With fix: PASS (mask correctly marks assistant tokens)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@zucchini-nlp (author of the original return_assistant_tokens_mask support in PR #38545, multimodal models)