Support scalar (non-channelwise) decay in the GDN kernels

[JakeStevens]

Summary:
channelwise_gated_delta_rule and gated_delta_net only accepted a per-channel decay gate ([B, Hkv, T, K]).

Qwen3-Next-style GatedDeltaNet produces one gate per head ([B, Hkv, T]), so it had no path to the optimized kernels and fell back to a Python token-by-token recurrence.

This offers a replacement for the previous reverted implentation that hooks into our optimized per-channel version.

Differential Revision: D114738016

[pytorch-bot]

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[meta-codesync]

@JakeStevens has exported this pull request. If you are a Meta employee, you can view the originating Diff in D114738016.

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