Fix grad getting enabled for parameters on which explicitly disabled in case of multiple optimizers

[jain-anshul]

What does this PR do?

In case of multiple optimizers, the bug causes grad to be enabled for all the parameters associated to an optimizer. This is erroneous in case there are any parameters which are explicitly kept non-trainable.

Fixed it by storing the original grad setting for all the parameters, and then setting it accordingly when that optimizer is called.

Co-authored by : @nisheethlahoti

Fixes #5292 <- this links related issue to this PR

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[codecov]

Codecov Report

Merging #5356 (b046f18) into master (389186c) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #5356   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         135     134    -1     
  Lines       10005    9979   -26     
======================================
- Hits         9339    9297   -42     
- Misses        666     682   +16     

[tchaton]

Would you mind adding a test ?

[tchaton]

Should be private.

[tchaton]

Dear @jain-anshul @nisheethlahoti,

Thanks a lot for your work ! I gave me the base to resolve the bug.

I solved in this PR: #5574

Closing this one for now.

Best,
T.C