Refactor the rest of pl_bolts.models.self_supervised

[akihironitta]

What does this PR do?

Follow-up of #338 #477 #479.

• Use pragma: no cover instead of pragma: no-cover. (see also: [coverage] Use no cover instead of no-cover #476)
• Place pragma: no cover at the beginning of clauses.
• Add missing __all__ to __init__.py files.
• Use if/else instead of try/except for optional import statements.

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• 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?

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[codecov]

Codecov Report

Merging #481 (78ab7d9) into master (a72766c) will decrease coverage by 0.95%.
The diff coverage is 65.11%.

@@            Coverage Diff             @@
##           master     #481      +/-   ##
==========================================
- Coverage   80.52%   79.57%   -0.96%     
==========================================
  Files         103      103              
  Lines        5710     5830     +120     
==========================================
+ Hits         4598     4639      +41     
- Misses       1112     1191      +79     

Flag	Coverage Δ
cpu	24.99% <30.23%> (-0.37%)	⬇️
pytest	24.99% <30.23%> (-0.37%)	⬇️
unittests	79.05% <51.16%> (-1.04%)	⬇️

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

Impacted Files	Coverage Δ
pl_bolts/models/self_supervised/cpc/transforms.py	93.40% <53.84%> (-6.60%)	⬇️
pl_bolts/models/self_supervised/moco/transforms.py	90.41% <56.25%> (-9.59%)	⬇️
..._bolts/models/self_supervised/simclr/transforms.py	78.26% <71.42%> (-2.05%)	⬇️
pl_bolts/models/self_supervised/__init__.py	100.00% <100.00%> (ø)
pl_bolts/models/self_supervised/cpc/__init__.py	100.00% <100.00%> (ø)
.../models/self_supervised/simclr/simclr_finetuner.py	13.55% <100.00%> (-86.45%)	⬇️
pl_bolts/models/self_supervised/swav/__init__.py	100.00% <100.00%> (ø)
...olts/models/self_supervised/swav/swav_finetuner.py	15.68% <100.00%> (-84.32%)	⬇️
pl_bolts/models/self_supervised/swav/transforms.py	63.63% <100.00%> (+0.96%)	⬆️
... and 3 more

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