gemm x86 support out_elemtype, multiheadattention and sdpa x86 support bf16 storage, skip mha bf16 tests

[nihui]

No description provided.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.10%. Comparing base (18a7ad1) to head (c87a5e1).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6623      +/-   ##
==========================================
+ Coverage   93.45%   94.10%   +0.65%     
==========================================
  Files         874      667     -207     
  Lines      280098   238244   -41854     
==========================================
- Hits       261758   224199   -37559     
+ Misses      18340    14045    -4295     

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[Copilot]

Pull request overview

This PR extends x86 compute paths to better support bf16 storage and Gemm output element type selection, and updates the test suite accordingly (including temporarily skipping MultiHeadAttention bf16 variants).

Changes:

• Add output_elemtype handling to the x86 bf16 Gemm implementation so bf16 inputs can produce fp32 outputs.
• Enable bf16 storage support flags for x86 MultiHeadAttention and SDPA, adjusting internal execution to accommodate bf16 storage.
• Add a new Gemm test (test_gemm_5.cpp) and update test utilities to skip MultiHeadAttention bf16 testing.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/testutil.cpp	Skips MultiHeadAttention bf16 tests; adds missing delete op on Vulkan skip paths (but early-return cleanup still incomplete).
tests/test_gemm_5.cpp	New Gemm test covering output_elemtype=fp32 across shapes/transposes.
src/layer/x86/sdpa_x86.cpp	Enables bf16 storage and updates intermediate/output allocations and memcpy sizes to respect bf16 elemsize.
src/layer/x86/multiheadattention_x86.cpp	Enables bf16 storage; forces certain sublayers to fp32 and adds a bf16→fp32 cast for V before qkv gemm.
src/layer/x86/gemm_x86.cpp	Threads output_elemtype through bf16 Gemm path and allocates/stores fp32 when requested.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated 10 comments.

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