Fix amp autocast

[awaelchli]

What does this PR do?

Fixes #6077

We had an incorrect yield logic for the context manager.
Modifications to the tests don't pass on master, validating the fix.

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[codecov]

Codecov Report

Merging #6080 (c40a462) into master (0b27147) will decrease coverage by 2%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #6080    +/-   ##
=======================================
- Coverage      93%     91%    -2%     
=======================================
  Files         160     160            
  Lines       11402   11403     +1     
=======================================
- Hits        10654   10380   -274     
- Misses        748    1023   +275