Option to save Optimizer in the save method

[jrzaurin]

Simple adjustment to add the option to save the optimizer when using the Trainer's save method

[codecov]

Codecov Report

Attention: Patch coverage is 58.16327% with 41 lines in your changes missing coverage. Please review.

Project coverage is 93.94%. Comparing base (8057360) to head (5b4bd9e).

Files	Patch %	Lines
...ed_training/_base_contrastive_denoising_trainer.py	48.64%	19 Missing ⚠️
...pervised_training/_base_encoder_decoder_trainer.py	17.39%	19 Missing ⚠️
pytorch_widedeep/training/_base_trainer.py	90.47%	2 Missing ⚠️
pytorch_widedeep/training/trainer.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   94.38%   93.94%   -0.44%     
==========================================
  Files         117      117              
  Lines        7445     7504      +59     
==========================================
+ Hits         7027     7050      +23     
- Misses        418      454      +36     

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