Rename collections.py to metrics_collections.py to avoid clashing with collections package

[tkupek]

What does this PR do?

Debugging the current version of the master does not work for me, as the torchmetrics collections.py clashes with the Python default collections package and I get this error:

ImportError: cannot import name 'OrderedDict' from partially initialized module 'collections' (most likely due to a circular import) (...\torchmetrics\collections.py)

I suggest to rename it to metric_collections.py to avoid this problem.

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[codecov]

Codecov Report

Merging #695 (4fac557) into master (c741149) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #695   +/-   ##
=====================================
  Coverage      95%    95%           
=====================================
  Files         166    166           
  Lines        6413   6413           
=====================================
+ Hits         6103   6105    +2     
+ Misses        310    308    -2