Make dependency of vissl optional

[ar90n]

What does this PR do?

This PR is motivated to achieve the second change in this comment. And this PR contains the following modifications.

• Make training_strategy, head, pretraining_transform in the arguments of __init__ of ImageEmbedder optional
• Add DefaultAdapter to the Embedding task which is inspired by the Image Classification task's one.
• Fix a test about integration with Fiftyone to work correctly
• Add test cases for only embedding without VISSL

Part of #1031

This PR is based on #1271. Because its target branch was wrong.
I'm sorry for my wrong operations. I misunderstood that it would be okay to change its target branch.

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[codecov]

Codecov Report

Merging #1276 (1dffc9d) into master (b88d1ba) will increase coverage by 0.00%.
The diff coverage is 92.45%.

@@           Coverage Diff           @@
##           master    #1276   +/-   ##
=======================================
  Coverage   91.55%   91.56%           
=======================================
  Files         284      285    +1     
  Lines       12729    12774   +45     
=======================================
+ Hits        11654    11696   +42     
- Misses       1075     1078    +3     

Flag	Coverage Δ
unittests	91.56% <92.45%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/image/embedding/strategies/default.py	88.88% <88.88%> (ø)
flash/core/utilities/imports.py	92.89% <100.00%> (+0.04%)	⬆️
flash/image/embedding/model.py	98.48% <100.00%> (+0.09%)	⬆️
flash/image/embedding/strategies/__init__.py	100.00% <100.00%> (ø)
...ash/image/embedding/strategies/vissl_strategies.py	100.00% <100.00%> (ø)
flash/text/question_answering/model.py	92.51% <0.00%> (+0.68%)	⬆️

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[ethanwharris]

Awesome 😃 Just a couple of minor comments

[ar90n]

This PR is ready for review. Please review it.
I have one question about dealing with DeepSource. Could I ignore its failure as same as other PRs?

I'm sorry for making a messy commit log. I struggled to solve CI issues.

[ethanwharris]

LGTM 😃 @ar90n Fine to ignore the deep source issues, we plan to get rid of it. No worries about the commit history! That's why we always squash and merge 😆

[ar90n]

@ethanwharris
Thanks for your review. I'm so glad to make this PR accepted. I'm looking forward to this PR being merged.

[ethanwharris]

@ar90n No worries, thanks for your work on this!! Very glad to see it all working smoothly 😃