CUDA: Fix loop unrolling for BW in mul_mat_q_stream_k_fixup (#19053)
By providing stride_* variables as size_t (i.e., 64-bit) the compiler can
correctly unroll the [two for-loops](https://github.com/ggml-org/llama.cpp/blob/
557515be1e93ed8939dd8a7c7d08765fdbe8be31/ggml/src/ggml-cuda/mmq.cuh#L3789-L3816)
on BW. This gives some perf for prefill/pp phase on BW, while not affecting
other SMs:
| GPU | Model | Test | t/s master | t/s osimons/fix_bw_mmq_fixup_kernel | Speedup |
|:--------------------------------------------------------|:----------------------|:-------|-------------:|--------------------------------------:|----------:|
| NVIDIA RTX 6000 Ada Generation | gpt-oss 20B MXFP4 MoE | pp8096 | 8404.05 | 8375.79 | 1.00 |
| NVIDIA RTX 6000 Ada Generation | llama 3B Q4_K_M | pp8096 | 16148.93 | 16019.60 | 0.99 |
| NVIDIA RTX 6000 Ada Generation | llama 8B Q4_0 | pp8096 | 8008.29 | 7978.80 | 1.00 |
| NVIDIA RTX 6000 Ada Generation | nemotron_h 9B BF16 | pp8096 | 4263.16 | 4248.53 | 1.00 |
| NVIDIA RTX 6000 Ada Generation | nemotron_h 9B Q4_K_M | pp8096 | 5165.11 | 5157.43 | 1.00 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | gpt-oss 20B MXFP4 MoE | pp8096 | 12582.80 | 12758.37 | 1.01 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | llama 3B Q4_K_M | pp8096 | 16879.10 | 17619.47 | 1.04 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | llama 8B Q4_0 | pp8096 | 10649.90 | 10982.65 | 1.03 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | nemotron_h 9B BF16 | pp8096 | 7717.73 | 7716.22 | 1.00 |
| NVIDIA RTX PRO 6000 Blackwell Max-Q Workstation Edition | nemotron_h 9B Q4_K_M | pp8096 | 7301.90 | 7370.38 | 1.01 |
overview / pkg / ggml / sources / llama.cpp / commit