Add normalize arg to LPIPS metric

[SkafteNicki]

What does this PR do?

Fixes #1214
Add support for the normalize argument in the backend of the lpips metric

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?

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?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1216 (c9c5506) into master (ebdaea7) will decrease coverage by 49%.
The diff coverage is 88%.

@@           Coverage Diff            @@
##           master   #1216     +/-   ##
========================================
- Coverage      86%     37%    -49%     
========================================
  Files         191     191             
  Lines       11312   11316      +4     
========================================
- Hits         9693    4207   -5486     
- Misses       1619    7109   +5490