[metrics] change default behaviour of state dict

[SkafteNicki]

What does this PR do?

Fixes
#4361
#4645
#4666

As default metrics add their internal state to the models state_dict. However, it seems counter intuitive to users (already 3 issues on this specific topic), and even though we since v1.0.6 provides a .persistent(mode) to change this behavior, the feedback indicates that it is probably better as default to not add the metric state to state_dict (and then the method can be used to switch it on).

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[codecov]

Codecov Report

Merging #4685 (7b60f4b) into master (be60efb) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4685   +/-   ##
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- Coverage      93%     93%   -0%     
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  Files         117     117           
  Lines        8932    8932           
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- Hits         8299    8298    -1     
- Misses        633     634    +1     

[teddykoker]

Makes sense to me, I think I overlooked this when we first implemented them

[rohitgr7]

LGTM. Just one comment.

[tchaton]

LGTM !