Example and documentation for LightningCLI linking model and data arguments

[mauvilsa]

What does this PR do?

It can be a common use case that both the model and data module classes require the same input parameter. This pull request updates the computer_vision_fine_tuning.py by using LightningCLI and linking parameters. Also the LightningCLI documentation is updated explaining this.

There is also a change in the LightningCLI methods. The linking must be done after adding all arguments. But the method to override for adding arguments was run before the add of core arguments. Furthermore it makes sense that in the same method arguments are added and links are configured, see computer_vision_fine_tuning.py#L276-L286. So the add_arguments_to_parser method was moved after the core arguments and the before_parse_arguments was removed since it was redundant.

The linking of arguments was briefly discussed in #6862.

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[codecov]

Codecov Report

Merging #7299 (d07f0f0) into master (393b252) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7299    +/-   ##
=======================================
- Coverage      91%     87%    -4%     
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  Files         200     200            
  Lines       12875   12873     -2     
=======================================
- Hits        11765   11240   -525     
- Misses       1110    1633   +523     

[awaelchli]

Maybe also bump the jsonargparse version? It's anyway new for 1.3. I Had to upgrade when running the example.