improve vit int8 mha opr

[tpoisonooo]

这个 PR 写了 mha 的 x86 简单优化：

• avx512f 指令集， ViT 从 670 -> 645ms
• avx2 指令集，ViT int8 从 1662 -> 1240ms

现在整个网络是 GEMM 比较慢

codebase 是 PR 3940 rebase 了一下 master，啥时候能 review 啊，感觉 diff 越来越大...

[lgtm-com]

This pull request introduces 4 alerts when merging e0a0ca6 into b4ba207 - view on LGTM.com

new alerts:

• 4 for Short global name

[codecov-commenter]

Codecov Report

Merging #4096 (9454c51) into master (b4ba207) will decrease coverage by 2.51%.
The diff coverage is 99.21%.

❗ Current head 9454c51 differs from pull request most recent head 42ad426. Consider uploading reports for the commit 42ad426 to get more accurate results

@@            Coverage Diff             @@
##           master    #4096      +/-   ##
==========================================
- Coverage   94.42%   91.91%   -2.52%     
==========================================
  Files         747      557     -190     
  Lines      178769   116257   -62512     
==========================================
- Hits       168811   106860   -61951     
+ Misses       9958     9397     -561     

Impacted Files	Coverage Δ
src/layer/x86/softmax_x86.cpp	98.13% <ø> (-0.08%)	⬇️
src/layer/x86/multiheadattention_x86.cpp	98.89% <98.89%> (ø)
src/layer/multiheadattention.cpp	98.91% <99.40%> (+5.68%)	⬆️
src/layer/x86/x86_usability.h	86.86% <100.00%> (-13.14%)	⬇️
src/layer/arm/convolution_winograd_transform.h	0.00% <0.00%> (-100.00%)	⬇️
...c/layer/arm/convolution_winograd_transform_bf16s.h	0.00% <0.00%> (-98.31%)	⬇️
src/layer/arm/flatten_arm.cpp	35.74% <0.00%> (-63.46%)	⬇️
src/layer/x86/convolution_3x3_pack1to4.h	49.13% <0.00%> (-50.87%)	⬇️
src/layer/arm/packing_arm.cpp	64.53% <0.00%> (-31.76%)	⬇️
src/layer/x86/quantize_x86.cpp	68.67% <0.00%> (-28.26%)	⬇️
... and 474 more

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[lgtm-com]

This pull request introduces 1 alert when merging 49cbb14 into 00c08d7 - view on LGTM.com

new alerts:

• 1 for FIXME comment

[lgtm-com]

This pull request introduces 1 alert when merging 9c1c2c9 into 00c08d7 - view on LGTM.com

new alerts:

• 1 for FIXME comment

[lgtm-com]

This pull request introduces 1 alert when merging 42ad426 into 00c08d7 - view on LGTM.com

new alerts:

• 1 for FIXME comment