Raise a MisconfigurationException when trainer functions are called with ckpt_path="best" but checkpoint_callback isn't configured

[rohitgr7]

What does this PR do?

Right now if checkpoint callback is not configured and you do trainer.test(ckpt_path='best') then it raises:

'NoneType' object has no attribute 'best_model_path'

this is a bit confusing maybe, so just added a check to look for checkpoint callback first

Does your PR introduce any breaking changes? If yes, please list them.

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[codecov]

Codecov Report

Merging #9841 (71e45e3) into master (f16bfe9) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9841    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         178     178            
  Lines       15647   15649     +2     
=======================================
- Hits        14502   13895   -607     
- Misses       1145    1754   +609     

[tchaton]

LGTM ! Nice catch :)

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>