PR #34173: [ROCm][XLA:GPU] Rename warp to shmem_group in PackedTranspose

Imported from GitHub PR https://github.com/openxla/xla/pull/34173

Rename `warp` to `shmem_group` in `PackedTranspose`. 

Also calculate their count as `kNumThreadsPerBlock / kNumShmemBanks` to avoid inconsistency when manually specified.

This change is NFC for any GPU in upstream. However, it fixes a performance regression in downstream for AMD GPUs caused by inconsistency between `shmem_group size`, `kNumThreadsPerBlock` and `kNumShmemBanks`. It ended up in a situation downstream where half of the launched threads per block were not utilized at all.

Update packed transpose tests to verify correct thread utilization.
Copybara import of the project:

--
390f1a7283327449f6319de6ada81b61d006b916 by Aleksei Nurmukhametov <anurmukh@amd.com>:

[XLA:GPU] Rename warp to shmem_group in PackedTranspose

Also calculate their count as kNumThreadsPerBlock / kNumShmemBanks to
avoid inconsistency when manually specified.

This change is NFC for any GPU in upstream. However, it fixes a
performance regression in downstream for AMD GPUs caused by
inconsistency between shmem_group size, kNumThreadsPerBlock and
kNumShmemBanks. It ended up in a situation downstream where half of the
launched threads per block were not utilized at all.

Update packed transpose tests to verify correct thread utilization.

Merging this change closes #34173

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/34173 from ROCm:anurmukh/fix-packed-transpose-threads 390f1a7283327449f6319de6ada81b61d006b916
