fix: use SM90ArchSpec instead of SM100ArchSpec in sm90_bf16_k_grouped_gemm

The function sm90_bf16_k_grouped_gemm was incorrectly using SM100ArchSpec
to calculate TMA descriptor block sizes. Since this file is the SM90
implementation, it should consistently use SM90ArchSpec like the other
functions in this file (sm90_bf16_gemm, sm90_m_grouped_bf16_gemm_contiguous,
etc.).

This fixes a copy-paste error that could cause incorrect block size
calculations on SM90 (Hopper) GPUs.

Fixes #242

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Co-Authored-By: Claude <noreply@anthropic.com>