[Feat] Adding PruningCallback

[tchaton]

What does this PR do?

This PR adds a pruning Callback.

Fixes # (issue) <- this links related issue to this PR

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

• In the file tests/callbacks/test_pruning.py:

Line 138:81: E203 whitespace before ','

Comment last updated at 2021-01-27 01:05:25 UTC

[codecov]

Codecov Report

Merging #5618 (9caef2e) into release/1.2-dev (7e2e874) will increase coverage by 0%.
The diff coverage is 90%.

@@               Coverage Diff                @@
##           release/1.2-dev   #5618    +/-   ##
================================================
  Coverage               89%     89%            
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  Files                  163     164     +1     
  Lines                12070   12218   +148     
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+ Hits                 10795   10928   +133     
- Misses                1275    1290    +15     

[SkafteNicki]

Just a high-level review...
Additional comments:

• there is an empty file pytorch_lightning/callbacks/pruning_utils/__init__.py that probably can be deleted
• insert link in the Built-in Callbacks section in the callbacks.rst doc file

[SeanNaren]

Excited to use this :)

[z-a-f]

LGTM, less some nits. Will leave it to other to approve

[Borda]

btw, https://medium.com/pytorch/pruning-with-catalyst-50e98f2cef2d