Add half precision testing [2/n]

[SkafteNicki]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Add half testing for the remaining regression metrics.

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

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[codecov]

Codecov Report

Merging #135 (9e21f63) into master (53d5701) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           master     #135     +/-   ##
=========================================
  Coverage   96.76%   96.76%             
=========================================
  Files         156       78     -78     
  Lines        4816     2410   -2406     
=========================================
- Hits         4660     2332   -2328     
+ Misses        156       78     -78     

Flag	Coverage Δ
Linux	79.29% <100.00%> (+0.01%)	⬆️
Windows	79.29% <100.00%> (+0.01%)	⬆️
cpu	96.76% <100.00%> (+<0.01%)	⬆️
gpu	?
macOS	96.76% <100.00%> (+<0.01%)	⬆️
pytest	96.76% <100.00%> (+<0.01%)	⬆️
python3.6	96.75% <100.00%> (+<0.01%)	⬆️
python3.8	96.76% <100.00%> (+<0.01%)	⬆️
python3.9	96.76% <100.00%> (+<0.01%)	⬆️
torch1.3.1	95.55% <100.00%> (+<0.01%)	⬆️
torch1.4.0	95.68% <100.00%> (+<0.01%)	⬆️
torch1.8.1	96.76% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...etrics/functional/regression/explained_variance.py	100.00% <100.00%> (ø)
torchmetrics/functional/regression/r2score.py	95.12% <100.00%> (+0.12%)	⬆️
.../torchmetrics/regression/mean_squared_log_error.py
...rics/functional/classification/hamming_distance.py
__w/1/s/torchmetrics/wrappers/__init__.py
.../1/s/torchmetrics/retrieval/retrieval_precision.py
__w/1/s/torchmetrics/classification/iou.py
...ics/functional/classification/average_precision.py
...1/s/torchmetrics/functional/classification/dice.py
...torchmetrics/functional/classification/__init__.py
... and 70 more

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[tchaton]

LGTM !