Backwards compatibility with the MPS support

[ahoust17]

This is a small fix which checks if torch.backends has an 'mps' attribute (older versions do not). There is still no way to check on GPU usage for the mps device, but this will surely be coming in the future.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (2d77fbd) 75.98% compared to head (0802e4a) 75.98%.
Report is 5 commits behind head on master.

Files	Patch %	Lines
atomai/trainers/trainer.py	50.00%	2 Missing ⚠️
atomai/utils/nn.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #83   +/-   ##
=======================================
  Coverage   75.98%   75.98%           
=======================================
  Files          71       71           
  Lines        7141     7141           
=======================================
  Hits         5426     5426           
  Misses       1715     1715           

Flag	Coverage Δ
unittests	75.98% <40.00%> (ø)

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