Trainer now calls on_load_checkpoint() when resuming from a checkpoint

[yukw777]

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What does this PR do?

Fixes #1190.

I don't like that the trainer takes a path to load from checkpoint that is different from LightningModule's load_from_checkpoint() method, but i thought this was the simplest way to fix it since I didn't want to get into the nitty gritty of the trainer logic. I'm sure there's a reason why it does things differently rather than using LightningModule's method.

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[codecov]

Codecov Report

Merging #1666 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1666   +/-   ##
======================================
  Coverage      88%     88%           
======================================
  Files          71      71           
  Lines        4175    4176    +1     
======================================
+ Hits         3692    3693    +1     
  Misses        483     483