[1/2] Use any task as an embedder for any layer

[ethanwharris]

What does this PR do?

This PR adds support for calling as_embedder on any task to convert it to an embedder. The embeddings retain gradient information so they could be used for training the model weights.

This approach uses an exception to break out of the forward in intermediate layers to avoid excess computation.

Still to come in follow-up: docs

Before submitting

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? - Coming in 2/2
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

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Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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[codecov]

Codecov Report

Merging #1396 (a280e7e) into master (3e4c8bb) will decrease coverage by 0.01%.
The diff coverage is 93.10%.

@@            Coverage Diff             @@
##           master    #1396      +/-   ##
==========================================
- Coverage   92.91%   92.90%   -0.02%     
==========================================
  Files         285      286       +1     
  Lines       12778    12836      +58     
==========================================
+ Hits        11873    11925      +52     
- Misses        905      911       +6     

Flag	Coverage Δ
unittests	92.90% <93.10%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/utilities/embedder.py	91.48% <91.48%> (ø)
flash/core/model.py	91.42% <100.00%> (+0.23%)	⬆️
flash/text/question_answering/model.py	93.19% <0.00%> (-1.37%)	⬇️

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[justusschock]

LGTM

[otaj]

LGTM!