Ameyn/gdn bf16 dispatcher and 4d pool (#3268)

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## Summary by CodeRabbit

* **Bug Fixes**
* Fixed BF16 decode to correctly handle padded/non-contiguous state
pools and to reuse caller-provided BF16 output buffers without
unnecessary copies.

* **Optimizations**
* Kernel compilation now differentiates pool memory layouts to produce
correct, efficient code for strided pools.

* **Tests**
* Added a CUDA regression test validating BF16 decoding with
padded/strided pool layouts.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>