Fix un-usability of manual optimization in Flash.

[karthikrangasai]

What does this PR do?

In a Flash Task, self.optimizers() and self.lr_schedulers() point to the respective Flash Registry instead of returning the optimizers and the lr schedulers used by the training job. This stands in the way of performing manual optimization.

This PR attempts to fix this issue.

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• Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #1342 (c832c8a) into master (6297566) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1342      +/-   ##
==========================================
- Coverage   92.68%   92.68%   -0.01%     
==========================================
  Files         286      286              
  Lines       12802    12802              
==========================================
- Hits        11866    11865       -1     
- Misses        936      937       +1     

Flag	Coverage Δ
unittests	92.68% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/core/model.py	91.19% <100.00%> (ø)
flash/text/question_answering/model.py	93.19% <0.00%> (-0.69%)	⬇️

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[justusschock]

LGTM! Can you also add an entry to the changelog?

[karthikrangasai]

Sure, I'll update it.

[karthikrangasai]

Hello @justusschock , could you please re-run the failed tests. They have failed because of some Omegaconf issue and I think my changes don't interfere with failure of the runs.

[ethanwharris]

LGTM 😃