fix setting batch_size attribute in batch_size finder (finishing PR #2523)

[awaelchli]

What does this PR do?

Follow up to #2523 (unfinished, stale PR)

Added test and made sure it's failing on master.

Fixes #2484

• Set model.batch_size based on model.hparams.batch_size if not defined
• Changed init_val for batch_size to 0 so that it starts with user defined batch_size instead of 2 all the time

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[awaelchli]

@SkafteNicki I finished this old PR and added tests as best as I could.
Not sure why the default argument needs to be changed.

[codecov]

Codecov Report

Merging #3043 into master will increase coverage by 29%.
The diff coverage is 86%.

@@           Coverage Diff            @@
##           master   #3043     +/-   ##
========================================
+ Coverage      57%     86%    +29%     
========================================
  Files          81      81             
  Lines        7585    7669     +84     
========================================
+ Hits         4358    6610   +2252     
+ Misses       3227    1059   -2168     

[mergify]

This pull request is now in conflict... :(