kernel_mul_mm_id splits its NR1 = 32 token tile into two 16-row halves and skips
the upper half when the expert did not fill it, on both the tensor and simdgroup
paths. The tB extents are corrected to (NK, NR1H) for the [NR1][NK] row-major tile.
The B tile is staged unconditionally, as on master: rows past nr1 restage a clamped
duplicate of a valid row, lie in the output-row dimension so they never contribute
to a valid row, and are dropped by the final store loop.
test-backend-ops: re-draw the expert ids between perf iterations of test_mul_mat_id
so MoE perf numbers are not warm-cache, and add token-tile boundary coverage using
n_used == n_mats, which routes every token to every expert so each expert receives
exactly n rows; n = 32, 33, 47, 48, 49 reach mul_mm_id and leave a last tile of 32,
1, 15, 16 and 17 rows.
Foad Abo Dahood
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