Fix SSIM memory

[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?

Fixes #529
Old strategy crates a list of tensors that is much longer than required for the computation. New strategy thus saves some memory.

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 #539 (148c2e3) into master (68e5636) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #539   +/-   ##
=====================================
- Coverage      95%    95%   -0%     
=====================================
  Files         132    132           
  Lines        4676   4676           
=====================================
- Hits         4457   4451    -6     
- Misses        219    225    +6     

[Borda]

@SkafteNicki "ready-to-go" is an automated label, so you cannot cheat it :D

[ethanwharris]

LGTM 😃

[SkafteNicki]

@SkafteNicki "ready-to-go" is an automated label, so you cannot cheat it :D

@Borda fair enough. Then I just create a "Nicki says this is ready to go" label :P