Multi-column on Intel XMX matrix engines refactored (#29864)
* Multi-column on Intel XMX matrix engines refactored
Multi-column (2 to 80 activation columns) mul_mat_vec_q for reordered K-quant and Q8_0 weights on the Intel XMX matrix engines (int8 DPAS, ESIMD), for the SYCL backend. It targets speculative decoding verification (2 to 8 tokens per step) and short prompt batches (9 to 80 tokens). Three types also use it for a single column (Q2_K, Q3_K, Q8_0).
* Fixed regression bug
real bug found: the first version produced NaN whenever the row count wasn't a multiple of 4
* Removed GGML_TYPE_Q8_0 activation
* rewritten xmx_mul_mat
64-byte alignment and 16-byte pitch check on the activations
* Implemented fused gate/up kernel
One launch computes up * act(gate) for both FFN weights, reading the activations once and applying SWIGLU or GEGLU in the epilogue. It covers up to 16 columns and works for all six XMX types.
* Removed the temporary XMX_NOGLU environment switch
* Q2_K min-sum optimization
Q2_K weight is d·scl·q − dmin·mc, where scl (the 4-bit scale) and mc (the 4-bit min code) apply to each half of a 32-weight group
* Added dispatch check and GRF sweep
tg128 on Q4_K_M goes from 26.67 to 28.23 tok/s on the B70 (+5.9%).
Q6_K is unchanged.
MUL_MAT passes 1727/1727 and MUL_MAT_VEC_FUSION 1361/1361.
Perplexity at batch 1 is 5.2064 with XMX and 5.2058 without.
* Renamed variables
* Added 2 environment variables
GGML_SYCL_XMX_MAX_COLS (default 80, any value 0 or higher) and GGML_SYCL_XMX_GLU_MAX_COLS (default 16, range 0 to 16): the largest batch for the fused gate/up kernel. It is clamped to 16 because that is the most the fused kernel is compiled for, so the variable can only lower it.
* Update esimd.hpp
1. All five quant structs were repeating the same boilerplate, so I moved it into shared pieces.
2. A new helper xmx_term does one DPAS, converts it to float and scales it per row. The four branches are now one or two lines each, and the shared Cb += z * d8r is written once.
The refactor passes all three op tests. MUL_MAT_VEC_FUSION is 1369/1369, MUL_MAT is 2281/2281 (the count includes your uncommitted f32 tests) and MUL_MAT_ID is 937/937.
