Simple aggregation metrics

[SkafteNicki]

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• Did you write any new necessary tests?

What does this PR do?

Fixes #505
Introduces a couple of simple aggregation metrics for aggregating simple streams of values.
As self.log in lightning do not accept user defined reduce_fx, it would be great if torchmetrics can supply some simple metrics for aggregating data. This PR implements MeanMetric (updated version of AverageMeter), SumMetric, MinMetric, MaxMetric and CatMetric that importantly also can deal with nan values (what started the issue).

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[codecov]

Codecov Report

Merging #506 (6461b90) into master (56dbfd2) will increase coverage by 0%.
The diff coverage is 99%.

@@          Coverage Diff          @@
##           master   #506   +/-   ##
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  Coverage      95%    95%           
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  Files         134    134           
  Lines        4809   4873   +64     
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+ Hits         4568   4632   +64     
  Misses        241    241           

[Borda]

@SkafteNicki does it also addresses #49?

[SkafteNicki]

@Borda no PR #556 does that :]

[tchaton]

LGTM !