Automatically check DataModule.has_{setup,teardown,prepare_data} [2/2]

[carmocca]

What does this PR do?

This PR automates checking whether the appropriate setup/teardown/prepare_data has been called. The check was only performed by the Trainer. Now it is done by the DataModule itself.

This has implications for the users if they were manually calling these functions themselves, as they will now be a no-op if they had been called already.

# user code:
dm.setup('fit')
...
dm.setup('fit')  # before this PR setup would get called it again, now it is a no-op

Fixes:

Fixes the check when the stage is None. Before it wasn't checking each attribute (has_setup_{fit,validate,test}) but has_setup_None which is not defined.

This PR does NOT fix_ #7286 because to properly resolve it, we need to avoid stage becoming None in the first place.

Note

The changes in this PR are the opposite of those discussed in #7301.

This PR attempts to fix the current design implementation. Not saying we should not change the design itself in the future.

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[codecov]

Codecov Report

Merging #7238 (fe91797) into master (3ec5420) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7238   +/-   ##
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- Coverage      92%     92%   -0%     
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  Files         199     199           
  Lines       13030   13034    +4     
======================================
- Hits        11967   11955   -12     
- Misses       1063    1079   +16     

[ananthsub]

Agreed with the fix for now but I'm in favor of simplifying/deprecating these properties in v1.4

[tchaton]

LGTM ! @ananthsub I think this is an important feature as preparing a dataset can be costly and shouldn't be done several times.

How do you propose to solve this problem if we remove those properties ?