Future proof LPIPs metric

[SkafteNicki]

What does this PR do?

Fixes #1546
Currently the LPIPs metric relies on the third party package https://github.com/richzhang/PerceptualSimilarity which is not maintained anymore. With the changes to torchvision on how models are loaded, this model will break in the future if we keep relying on the third party package (currently a warning is raised).
This PR copies in the relevant code and updates it to be compatible with torchvision 0.13 and higher.

I have included the required copyright for copying the code.

Before submitting

• Was this discussed/agreed 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?

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[lantiga]

Approved pending addition of license notice (see comment).

[stancld]

Some suggestions to the copied code.

[codecov]

Codecov Report

Merging #1575 (f750a0e) into master (eeea871) will increase coverage by 0%.
The diff coverage is 96%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #1575    +/-   ##
=======================================
  Coverage      87%     88%            
=======================================
  Files         240     241     +1     
  Lines       13510   13705   +195     
=======================================
+ Hits        11809   12001   +192     
- Misses       1701    1704     +3