fix the problem of sigmoid gradient generating NaN

[wcshds]

Pull Request Template

Checklist

• [x] Confirmed that run-checks all script has been executed.
• [x] Made sure the book is up to date with changes in this PR.

Related Issues/PRs

fix #1139

Changes

Use sigmoid's derivative formula directly to avoid differentiating log and exp in autodiff.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (76c9358) 85.67% compared to head (197cf27) 85.95%. Report is 4 commits behind head on main.

Files	Patch %	Lines
burn-autodiff/src/ops/activation.rs	95.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1140      +/-   ##
==========================================
+ Coverage   85.67%   85.95%   +0.27%     
==========================================
  Files         513      518       +5     
  Lines       57006    57724     +718     
==========================================
+ Hits        48841    49616     +775     
+ Misses       8165     8108      -57     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.