sign operator

[brightening-eyes]

this is one of the simplest things in onnx which I thaught to implement on ncnn.
the same is true with softsign and so on.

[codecov-commenter]

Codecov Report

Merging #5042 (4d41a89) into master (a12a14f) will decrease coverage by 5.21%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #5042       +/-   ##
===========================================
- Coverage   94.72%   89.52%    -5.21%     
===========================================
  Files         764      252      -512     
  Lines      229616    67839   -161777     
===========================================
- Hits       217513    60733   -156780     
+ Misses      12103     7106     -4997     

Files Changed	Coverage Δ
src/layer/sign.cpp	100.00% <100.00%> (ø)

... and 676 files with indirect coverage changes

[nihui]

sign should be placed in unaryop

[brightening-eyes]

I'm doing it like onnxruntime.
I suppose its easier to implement and maintain than unaryop. especially when I do not have experience with loongarch stuff.