Change DDPPlugin to DDPStrategy

[ejm714]

DDPPlugin was deprecated in pytorch lightning 1.8 which came out today. Replace with supported DDPStrategy. We don't pin PTL, so we are subject to deprecated code in new releases.

Bonus fix:

• address checkpoint warning

2022-11-01 19:42:05.757 | INFO     | zamba.models.model_manager:train_model:331 - Calculating metrics on validation set.
/usr/local/lib/python3.8/dist-packages/pytorch_lightning/trainer/connectors/checkpoint_connector.py:129: UserWarning: `.validate(ckpt_path=None)` was called without a model. The best model of the previous `fit` call will be used. You can pass `.validate(ckpt_path='best')` to use the best model or `.validate(ckpt_path='last')` to use the last model. If you pass a value, this warning will be silenced.

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[codecov-commenter]

Codecov Report

Merging #244 (12ddfe5) into master (bb89011) will not change coverage.
The diff coverage is 100.0%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #244   +/-   ##
======================================
  Coverage    87.0%   87.0%           
======================================
  Files          28      28           
  Lines        1962    1962           
======================================
  Hits         1707    1707           
  Misses        255     255           

Impacted Files	Coverage Δ
zamba/models/model_manager.py	84.3% <100.0%> (ø)
zamba/pytorch_lightning/utils.py	97.4% <100.0%> (ø)