opencl: ragged-tile MoE prefill FP16 GEMM optimization (skip padded expert tiles) (#25433)

• opencl: ragged-tile MoE prefill GEMM (skip padded expert tiles)

The MoE prefill GEMM groups tokens into TILESIZE_N=32 per-expert tiles; at low tokens-per-expert most tiles are mostly padding. When a tile's upper 16 slots are all padding (router index 0xFFFFFFFF), skip the second dotx16_reduce8 half. Numerically identical (skipped lanes are padding). Applied to all eight *_f32_ns MoE GEMMs; default on, opt out with GGML_OPENCL_MOE_RAGGED_FP16=0.

• opencl: quarter-granularity ragged MoE tile-skip (8-col skip-groups)

Replace the two half-tile dotx16_reduce8 calls in the 8 *_f32_ns MoE GEMMs with four dotx8_reduce4 (8-column) calls, skipping each empty trailing skip-group independently. Padding is always trailing, so the kernel rounds the valid count up to the skip granularity and skips fully-padding groups. Byte-identical to the non-skipped path. New env GGML_OPENCL_MOE_RAGGED_GRAN={8,16,32} (quarter/half/ off); default quarter.

• opencl: move ragged moe env var in cl_init

Co-authored-by: Li He [email protected]