Load ckpt path when model provided in validate/test/predict

[SeanNaren]

What does this PR do?

Closes #8347

Behaviour now is for test, validate and predict:

trainer.test(model, ckpt_path=None) # use provided model
trainer.test(model, ckpt_path='best') # load best model
trainer.test(model, ckpt_path='my_path') # load path

trainer.fit(model)
# then
trainer.test(ckpt_path=None) # load best model
trainer.test(ckpt_path='my_path') # load path

The biggest change is that if anyone did trainer.test(ckpt_path=None) to load the latest model, they would now need to change this to trainer.test(model) and pass the model reference directly (same for validate/predict).

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[codecov]

Codecov Report

Merging #8352 (33f8917) into master (b256d6a) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8352   +/-   ##
======================================
  Coverage      92%     92%           
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  Files         218     218           
  Lines       14398   14407    +9     
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+ Hits        13296   13305    +9     
  Misses       1102    1102           

[tchaton]

LGTM !