LightningCLI support for argument links applied on instantiation

[mauvilsa]

What does this PR do?

From discussions mostly in slack it became clear that there are common cases in which parameters that are only available after instantiating the data module class are needed to instantiate the model. To achieve the user had to subclass LightningCLI in a non-trivial way. With this PR these cases can be implemented in a much simpler way. The concept of argument linking is extended so that they can be applied on instantiation. It use is explained in the change of lightning_cli.rst.

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[pep8speaks]

Hello @mauvilsa! Thanks for updating this PR.

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

Comment last updated at 2021-06-09 19:33:48 UTC

[codecov]

Codecov Report

Merging #7895 (42c6619) into master (b214442) will decrease coverage by 7%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #7895     +/-   ##
========================================
- Coverage      91%     84%     -7%     
========================================
  Files         204     204             
  Lines       13630   13657     +27     
========================================
- Hits        12414   11415    -999     
- Misses       1216    2242   +1026     

[carmocca]

Still draft? What's missing?

[awaelchli]

smart!