Call any trainer function from the LightningCLI

[carmocca]

What does this PR do?

Fixes #7226

Pitch

# LightningCLI(..., run=False)
python script.py ...  # does not run anything but parses arguments

# LightningCLI(..., run=True)  # default
python script.py fit ...  # current behaviour
python script.py tune ...
python script.py validate ...
python script.py test ...
python script.py predict ...

where {fit,tune,validate,test,predict} are all the available subcommands.

The implementation needs to allow implementing custom subcommands. This will be useful in Flash to implement the finetune subcommand.

A follow-up PR (out of scope for this one) could add subcommands which chain commands. Some potential examples would be tune-and-fit, fit-and-validate, or fit-and-test.

Breaking changes

This change will be breaking for all previous CLI users. They will now need to pass the fit subcommand to keep the old behaviour. Any users who were adding subcommand already will need to update their position too.

Before submitting

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PR review

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[pep8speaks]

Hello @carmocca! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-08-10 15:10:38 UTC

[codecov]

Codecov Report

Merging #7508 (620703b) into master (b5fb49a) will increase coverage by 0%.
The diff coverage is 99%.

@@          Coverage Diff           @@
##           master   #7508   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files         176     176           
  Lines       14804   14857   +53     
======================================
+ Hits        13032   13084   +52     
- Misses       1772    1773    +1     

Old review

[carmocca]

This is ready to go again, just waiting on a jsonargparse release with omni-us/jsonargparse#88 fixed (minor fix)

Pinging reviewers: @tchaton @ethanwharris @mauvilsa @Borda @awaelchli (dismissed old reviews)

[tchaton]

Looks neat !