cuda: exclude sm_75 from the short-query in-place quantized-K/V MMA FA

[umbertov]

Overview

Fixes the Turing GPU failure reported in #348.

In ggml_cuda_flash_attn_ext_mma_f16_switch_ncols1, skip the F16 MMA path on Turing GPUs (sm_75). That kernel does not support these GPUs, and using it can make prompt processing fail with the following message:

ERROR: CUDA kernel flash_attn_ext_f16 has no device code compatible with CUDA arch 750. ggml-cuda.cu was compiled for: 750,800,860,890,900,1200,1210

On sm_75, this keeps dispatch on the existing compatible path for this case.

Additional information

This change does not add a new Flash Attention kernel or enable the MMA path on Turing.

I have tested this change on my machine with RTX 2080 ti with the following command line invocation:

./build/bin/llama-server \
  -hf prism-ml/Ternary-Bonsai-2-27B-gguf:TQ1_0 \
  --jinja \
  -ngl 99 \
  -c 131072 \
  --ubatch-size 512 \
  -fa on \
  -ctk q4_0 \
  -ctv q4_0 \
  --port 8080 \
  --temp 1.0 --top-p 0.95 --top-k 20 --min-p 0.00

Requirements

• I have read and agree with the contributing guidelines: yes
• AI usage disclosure: yes. AI supported the codebase exploration which led to the code change. I reviewed the change and am responsible for it. This PR was not automatically generated.