[tox fix] Reject and skip: BufferLoad does not support MX layout

[mengzcai]

Motivation

BufferLoad does not support MX layout on gfx1250. Without this rejection, invalid kernel configurations are
generated, leading to runtime failures.

Technical Details

• Solution.py: Added a reject guard for non-TDM kernels on gfx1250 when MXBlockA or MXBlockB is enabled
  with BufferLoad.
• YAML test configs: Added skip-gfx1250 to 5 StreamK MX test files under Tests/common/streamk/gfx1250/ to skip these
  tests on gfx1250 until BufferLoad + MX layout is supported.

Test Plan

• Verified that MX + BufferLoad kernels on gfx1250 are correctly rejected during solution generation
• Confirmed non-MX and TDM kernels are unaffected
• StreamK MX tests (sk_mx*_quick.yaml) are skipped on gfx1250 in tox

Test Result

The following BufferLoad+MX yamls will be skipped in gfx1250 tox:
Tensile/Tests/common/streamk/gfx1250/sk_mxf6b8gemm_quick.yaml
Tensile/Tests/common/streamk/gfx1250/sk_mxf8b6gemm_quick.yaml
Tensile/Tests/common/streamk/gfx1250/sk_mxf8f4gemm_quick.yaml
Tensile/Tests/common/streamk/gfx1250/sk_mxf4gemm_quick.yaml
Tensile/Tests/common/streamk/gfx1250/sk_mxf8gemm_quick.yaml

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.