Remove momentum updating from val step and add separate val queue

[maximzubkov]

What does this PR do?

This pull request fixes the problem discussed in the following issues #615 #630

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• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• 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?
• 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?

Changes:

• Remove _momentum_update_key_encoder and _dequeue_and_enqueue from forward
• Add separate val_queue and val_queue_ptr
• Return key form forward

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

[codecov]

Codecov Report

Merging #631 (6d31284) into master (0045e64) will decrease coverage by 0.39%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
- Coverage   72.64%   72.25%   -0.40%     
==========================================
  Files         119      119              
  Lines        7352     7356       +4     
==========================================
- Hits         5341     5315      -26     
- Misses       2011     2041      +30     

Flag	Coverage Δ
cpu	72.25% <100.00%> (-0.40%)	⬇️
pytest	72.25% <100.00%> (-0.40%)	⬇️

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

Impacted Files	Coverage Δ
..._bolts/models/self_supervised/moco/moco2_module.py	58.82% <100.00%> (+0.99%)	⬆️
pl_bolts/datasets/cifar10_dataset.py	72.04% <0.00%> (-25.81%)	⬇️
pl_bolts/datasets/base_dataset.py	81.39% <0.00%> (-13.96%)	⬇️

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[Borda]

@ananyahjha93 mind check

[akihironitta]

@maximzubkov Thank you for sending the fix! LGTM!

@ananyahjha93 Could you double-check the change here?