Validate the model input of trainer methods

[awaelchli]

What does this PR do?

Fixes #13355
Partially addresses #13981

When passing something other than a LightningModule to the Trainer, we currently get a misleading error message that doesn't help the user find the issue. Now I'm adding a check to all the Trainer methods that need a LightningModule.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @kaushikb11 @awaelchli @ananthsub @ninginthecloud @jjenniferdai @rohitgr7 @akihironitta

[codecov]

Codecov Report

Merging #13892 (b0e6906) into master (9651e42) will increase coverage by 15%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #13892     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         341      341             
  Lines       26523    26575     +52     
=========================================
+ Hits        16184    20230   +4046     
+ Misses      10339     6345   -3994     

[tchaton]

LGTM !