[CK_TILE] Fix alignment in Stream-K workspace buffer (#3625)

* Fix alignment issue in Stream-K workspace buffer

In CK Tile Stream-K, the workspace buffer is used to hold flags and
partials, where the first i bytes holds the flags and the remaining
bytes hold partials. This change adds padding to the flags prefix of the
workspace buffer to ensure the number of bytes is 128B-aligned. Without
this alignment, since workgroups do not skip cache when reading from
partials, they may read stale partials data in cache, leading to
incorrect results. The added padding avoids the stale data reading.

This change also re-enables the test_ck_tile_streamk_reduction tests.

* Compute reference GEMM on GPU for test verification to decrease testing time