Fix three -Wunused-variable sites in HIP-built sources

[q10]

Summary:
Fixes the -Wunused-variable diagnostics in HIP-built sources that are not
demoted by a -Wno-error= and therefore become hard errors once -Werror is
restored on HIP.

Each of the three needed a different fix, because "unused variable" meant something
different in each case. None of them are actually dead code in every configuration.

1. moe/index_shuffling.cu -- storage_factor -> [[maybe_unused]]

It is used on CUDA, by the num_tokens_per_tile computation, but that block sits
under #ifndef USE_ROCM; the ROCm branch keys off num_experts instead. Meanwhile
the ROCm DISPATCH_E_* macros take the value as parameter S and discard it. So the
variable is genuinely unused on ROCm and genuinely used on CUDA.

Marked rather than moved or #ifdef-ed: both paths still name it as a macro
argument, so relocating the declaration would mean duplicating it or widening the
guard around unrelated code.

2. quantize/quantize.cu -- ret moved inside #ifndef USE_ROCM

Declared just above the guard, but its only assignments and its only read
(use_shmem = ret == cudaSuccess) are all inside #ifndef USE_ROCM; the ROCm branch
just sets use_shmem = false. The declaration belongs in the block. This is a pure
move -- no [[maybe_unused]] needed, and the CUDA path is untouched.

3. ck_extensions/bf16_grouped/bf16_grouped_gemm.hip -- dead K removed

int64_t K = B.size(2); with no reader. M and N next to it are both used; K
only appears in TORCH_CHECK message strings, which are string literals and do not
reference the variable. HIP-only file, so no dual-path concern -- deleted.

This is partial. The census that scoped this covered only 13% of the build before
the ROCm job timed out, so more -Wunused-variable sites may appear once it
completes.

Reviewed By: cthi

Differential Revision: D115963368

[meta-codesync]

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