[ruixiang63]

What does this PR do?

This PR removes unnecessary pageable Host-to-Device copies in the pure-PyTorch fallback for Gated DeltaNet (torch_chunk_gated_delta_rule and torch_recurrent_gated_delta_rule), used by Qwen3-Next, Qwen3.5, Qwen3.5 MoE and OLMo-Hybrid when flash-linear-attention is not installed.

The current implementation initializes last_recurrent_state and core_attn_out via torch.zeros(...).to(value).

Because torch.zeros(...) is called without a device argument, the tensor is first allocated in pageable host memory and zero-filled on CPU, then .to(value) triggers: pageable H2D copy and an implicit synchronization (.to() defaults to non_blocking=False).
e.g. In nsys traces it looks like following:

This PR allocates these tensors directly on the target device and dtype, eliminating the H2D copies and the implicit syncs entirely.

• Fixes a real perf regression observable in Nsight Systems traces of end-to-end training.
• Affects 4 model families (Qwen3-Next / Qwen3.5 / Qwen3.5-MoE / OLMo-Hybrid), all of which inherit this fallback.

Nsight system traces:

• Without this PR:

• With this PR:

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cc @Cyrilvallez @zucchini-nlp @ArthurZucker

[Cyrilvallez]

LGTM, it's indeed always better to initialize directly rather than calling to

[github-actions]

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

run-slow: olmo_hybrid, qwen3_5, qwen3_5_moe, qwen3_next

[ruixiang63]

@Cyrilvallez Thanks for the approval! It looks like there are still two workflows awaiting maintainer approval, and doc_build_status_check is still pending. Could you please approve/run them when you get a chance? Thanks!

[HuggingFaceDocBuilderDev]

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