Simplify backbone_image_classifier example

[mauvilsa]

What does this PR do?

In a slack discussion about LightningCLI it was suggested that untyped parameters should be added to the parser as type Any instead of being silently skipped. With the latest version of jsonargparse this is the behavior, however, this means that in the current backbone_image_classifier example the argument model.backbone will be added twice causing a failure. There is a temporal patch in jsonargparse so that this example does not fail, but this will be removed in the next version. The backbone_image_classifier has to change due to this, but also it can be made simpler as proposed here.

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[codecov]

Codecov Report

Merging #7246 (187c15d) into master (7a48db5) will decrease coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #7246    +/-   ##
=======================================
- Coverage      91%     87%    -4%     
=======================================
  Files         199     199            
  Lines       12799   12799            
=======================================
- Hits        11701   11147   -554     
- Misses       1098    1652   +554     

[mauvilsa]

@carmocca this change is according to our discussion in slack.

[tchaton]

LGTM !

[carmocca]

this means that in the current backbone_image_classifier example the argument model.backbone will be added twice causing a failure.

Why is this the case?
edit: nvm, realized it's due to

    def add_arguments_to_parser(self, parser):
        parser.add_class_arguments(Backbone, 'model.backbone')