[Qwen3_5]Remove unnecessary masked_fill_ in torch_chunk_gated_delta_rule attention computation: "attn = (q_i @ k_i.transpose(-1, -2) * decay_mask[:, :, i]).masked_fill_(mask, 0)"

[ENg-122]

What does this PR do?

Remove unnecessary masked_fill_(mask, 0) call in torch_chunk_gated_delta_rule.

The decay_mask computed earlier already encodes the causal/lower-triangular structure (upper-triangle values are zero), so masking the attention scores again with masked_fill_(mask, 0) is redundant and adds unnecessary overhead.

Before:
attn = (q_i @ k_i.transpose(-1, -2) * decay_mask[:, :, i]).masked_fill_(mask, 0)

After:
attn = (q_i @ k_i.transpose(-1, -2) * decay_mask[:, :, i])

• [√] I confirm that this is not a pure code agent PR.

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? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• [×] Did you write any new necessary tests?

Who can review?

@ArthurZucker @Cyrilvallez

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: olmo_hybrid, qwen3_5, qwen3_5_moe, qwen3_next