[Docs] fix on_after_backward example

[rohitgr7]

What does this PR do?

Fixes #5238

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[codecov]

Codecov Report

Merging #5278 (4b3f146) into master (1ec1d3e) will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #5278    +/-   ##
=======================================
- Coverage      93%     93%    -0%     
=======================================
  Files         135     135            
  Lines       10015    9875   -140     
=======================================
- Hits         9338    9185   -153     
- Misses        677     690    +13     

[Borda]

@rohitgr7 it seems it does only formating, do not touches any docs...

[rohitgr7]

@Borda it was grads = v before which is wrong, changed it to grads = v.grad.

[rohitgr7]

@williamFalcon mind review