Refactor pl_bolts.callbacks

[akihironitta]

What does this PR do?

Follow-up PR of #338.

• adds missing imports and __all__ in pl_bolts/callbacks/__init__.py
• defines _WANDB_AVAILABLE in pl_bolts/utils/__init__.py
• uses _*_AVAILABLE variables defined in Refactor pl_bolts.datamodules and pl_bolts.datasets #338 for optional package imports
• uses pragma: no cover instead of pragma: no-cover

If the above changes are too many in a single PR, I'll split this PR into some.

Before submitting

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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 #477 (06914df) into master (a72766c) will increase coverage by 0.11%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #477      +/-   ##
==========================================
+ Coverage   80.52%   80.63%   +0.11%     
==========================================
  Files         103      102       -1     
  Lines        5710     5769      +59     
==========================================
+ Hits         4598     4652      +54     
- Misses       1112     1117       +5     

Flag	Coverage Δ
cpu	25.29% <77.77%> (-0.07%)	⬇️
pytest	25.29% <77.77%> (-0.07%)	⬇️
unittests	80.20% <72.22%> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
pl_bolts/callbacks/data_monitor.py	89.28% <66.66%> (-1.46%)	⬇️
pl_bolts/callbacks/variational.py	95.65% <80.00%> (-2.03%)	⬇️
pl_bolts/callbacks/__init__.py	100.00% <100.00%> (ø)
pl_bolts/callbacks/ssl_online.py	96.29% <100.00%> (-3.71%)	⬇️
pl_bolts/utils/__init__.py	100.00% <100.00%> (ø)

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