Fix variable-length linear attention in Qwen3.5/Qwen3-Next
Author: Charly21r
Pull request: yes

# What does this PR do?

Fixes #46851 

Fixes the silent miscomputation of Qwen3.5 linear attention on packed / variable-length inputs.

`Qwen3_5GatedDeltaNet` only recovered packed-sequence boundaries from an explicit `cu_seq_lens_q` kwarg and never derived them from `position_ids`. Since downstream packages (e.g. Verl) assume the same interface as every other HF model (pass `position_ids`, boundaries are inferred), `Qwen3.5` silently produced incorrect results, with no error, whenever `cu_seqlens` was omitted. On top of that, the torch fallbacks (the chunked gated-delta-rule kernel and the short causal conv) ignored `cu_seqlens` entirely, so results were wrong even when the kwargs were passed and the FLA/causal-conv kernels weren't installed.

There were three cross-boundary leaks in the gated-delta net, this PR closes all of them:

1. Boundary derivation: `maybe_set_cu_seqlens_for_packing` recovers `cu_seq_lens`/`max_length` from packed `position_ids` in the text model forward when packing is detected and they weren't supplied, mirroring the standard Flash-Attention path (_is_packed_sequence / prepare_fa_kwargs_from_position_ids). Passing `position_ids` alone now works, like other models.
2. Chunked delta-rule torch fallback: `torch_chunk_gated_delta_rule` now processes each packed sub-sequence independently so neither attention nor the recurrent state leaks across boundaries.
3. Causal conv: `seq_idx_from_cu_seqlens` feeds the fused causal_conv1d_fn, and causal_conv1d_packed_torch convolves each sub-sequence independently in the torch fallback, so the kernel window can't mix tokens across boundaries.

Everything lives in the shared `modular_qwen3_next.py` / `modular_qwen3_5.py`, so the fix also covers Qwen3-Next and Qwen3.5-MoE.


## Code Agent Policy

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## Who can review?

@vasqu @ArthurZucker

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

run-slow: qwen3_5, qwen3_5_moe, qwen3_next

**CI Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=46859)

Closing as per https://github.com/huggingface/transformers/issues/46851#issuecomment-4787309390. Also, this is clearly a code agent PR; be careful with those since they're often unhelpful and can get you blocked if they start wasting maintainer time. We're able to run code agents ourselves, we don't need users to dive bomb random issues with them!