add option to log momentum

[jeremyjordan]

What does this PR do?

Adds the ability to optionally log momentum values in the LearningRateMonitor.

I followed the style of the existing code, except instead of saving all momentum values I decided to only save the last value. It seemed like this attribute was only being used for testing so I decided to keep the memory footprint lower.

Fixes #4354

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[codecov]

Codecov Report

Merging #4384 into master will increase coverage by 1%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4384   +/-   ##
======================================
+ Coverage      92%     93%   +1%     
======================================
  Files         111     111           
  Lines        8139    8060   -79     
======================================
- Hits         7477    7470    -7     
+ Misses        662     590   -72