fix apex gradient clipping

[ruotianluo]

What does this PR do?

Fixes #2556

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[ruotianluo]

Changed the clip_gradients input. not sure if there is any better way.

[codecov]

Codecov Report

Merging #2829 into master will decrease coverage by 0%.
The diff coverage is 91%.

@@          Coverage Diff           @@
##           master   #2829   +/-   ##
======================================
- Coverage      90%     90%   -0%     
======================================
  Files          78      78           
  Lines        6921    6929    +8     
======================================
+ Hits         6238    6245    +7     
- Misses        683     684    +1