viable/strict/1788331551: [ROCm][CK] Remove host-side FMHA sequence padding workaround (#195498)

Release notes describe removing host-side sequence padding workarounds for CK FMHA, passing original logical tensors directly to CK, and expanding operator coverage for unaligned sequences and related features. The change targets unpadded Q/K/V sequences and eliminates external padding and related scratch/outputs in the PyTorch CK wrapper. It includes test coverage across multiple ROCm architectures and configurations.

What changed: The external Q/K/V sequence padding, O/LSE scratch allocation, and output copies from the PyTorch CK FMHA wrapper were removed. The wrapper now forwards the original logical tensors to CK, which now rejects unpadded, unsafe instances via its generated dispatcher. Operator coverage was added for unaligned sequence lengths, dense bias, dropout replay, and grouped-query attention.

Why it matters: This simplifies the FMHA path and reduces host-side padding complexity, potentially improving performance and reliability on ROCm GPUs. The release also broadens test coverage to ensure correct behavior with unaligned sequences and related features.

Source: Release notes from pytorch — https://github.com/pytorch/pytorch/releases/tag/viable%2Fstrict%2F1788331551