[Qwen3.5] Fix Qwen3.5 linear attention multi-token cached forward

[kashif]

What does this PR do?

The gated-delta-net forward only used the cached recurrent state when seq_len == 1. For any multi-token forward with a populated cache (e.g. chunked prefill continuation or speculative-decoding verification), it fell through to chunk_gated_delta_rule(initial_state=None), silently restarting the linear layers from zero and ignoring the prefill state.

This breaks the causal-LM invariant that the logits at position i must not depend on whether later tokens are batched into the same call — position 0 of a 16-token verify forward ended up differing from the corresponding single-token cached decode, collapsing to high-frequency context tokens and destroying speculative-decoding correctness.

Add a use_cached_chunk path that, when has_previous_state is true and seq_len > 1:

• reads the cached conv_state / recurrent_state,
• prepends the conv context onto the chunk input so the causal conv sees the correct left-context,
• drops the prepended context from the output,
• passes the cached recurrent_state as initial_state to chunk_gated_delta_rule.

The same fix propagates to qwen3_5_moe via the modular system.

Add a unit test that compares the first-position output of a multi-token cached forward against the single-token cached forward on the same token and cache. Without this fix the mismatch is 100%.

Fixes # (issue)

Code Agent Policy

The Transformers repo is currently being overwhelmed by a large number of PRs and issue comments written by
code agents. We are currently bottlenecked by our ability to review and respond to them. As a result,
we ask that new users do not submit pure code agent PRs at this time.
You may use code agents in drafting or to help you diagnose issues. We'd also ask autonomous "OpenClaw"-like agents
not to open any PRs or issues for the moment.

PRs that appear to be fully agent-written will probably be closed without review, and we may block users who do this
repeatedly or maliciously.

This is a rapidly-evolving situation that's causing significant shockwaves in the open-source community. As a result,
this policy is likely to be updated regularly in the near future. For more information, please read CONTRIBUTING.md.

• 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?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[github-actions]

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

run-slow: qwen3_5, qwen3_5_moe

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.