Fix autocast in SSIM metric

[SkafteNicki]

What does this PR do?

Fixes #1453
Exchange ValueError with explicit dtype casting to support mixed precision autocast

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 #1454 (6e7cb97) into master (662a2a2) will decrease coverage by 51%.
The diff coverage is 0%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1454     +/-   ##
========================================
- Coverage      90%     39%    -51%     
========================================
  Files         211     211             
  Lines       10839   10839             
========================================
- Hits         9718    4228   -5490     
- Misses       1121    6611   +5490