multiheadattention attn mask

[nihui]

No description provided.

[codecov-commenter]

Codecov Report

Merging #4668 (1dd1a77) into master (b640574) will increase coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4668      +/-   ##
==========================================
+ Coverage   94.44%   94.60%   +0.16%     
==========================================
  Files         686      686              
  Lines      207186   207602     +416     
==========================================
+ Hits       195671   196408     +737     
+ Misses      11515    11194     -321     

Impacted Files	Coverage Δ
src/layer/arm/multiheadattention_arm.cpp	100.00% <100.00%> (ø)
src/layer/multiheadattention.cpp	94.15% <100.00%> (+0.44%)	⬆️
src/layer/vulkan/multiheadattention_vulkan.cpp	98.54% <100.00%> (+0.08%)	⬆️
src/layer/x86/multiheadattention_x86.cpp	100.00% <100.00%> (ø)

... and 51 files with indirect coverage changes