spectrogram via unfold + gemm

[futz12]

Use Gemm + unfold to speed spectrogram on x86 and vulkan

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.75000% with 40 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.22%. Comparing base (d7d6615) to head (55d961a).

Files with missing lines	Patch %	Lines
src/layer/vulkan/spectrogram_vulkan.cpp	89.08%	19 Missing ⚠️
src/layer/vulkan/inversespectrogram_vulkan.cpp	94.85%	9 Missing ⚠️
src/layer/x86/inversespectrogram_x86.cpp	95.36%	7 Missing ⚠️
src/layer/x86/spectrogram_x86.cpp	95.93%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6542      +/-   ##
==========================================
+ Coverage   93.21%   93.22%   +0.01%     
==========================================
  Files         848      849       +1     
  Lines      266689   266987     +298     
==========================================
+ Hits       248592   248902     +310     
+ Misses      18097    18085      -12     

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