[WIP] Remove the requirement for double update in forward

[tchaton]

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?

This PR optimizer the forward method of the Metric base class to prevent performing 2 updates.

Parts of #344

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 #612 (dbb480a) into master (c0e4250) will decrease coverage by 25%.
The diff coverage is 38%.

@@           Coverage Diff           @@
##           master   #612     +/-   ##
=======================================
- Coverage      95%    70%    -25%     
=======================================
  Files         166    166             
  Lines        6795   6813     +18     
=======================================
- Hits         6469   4795   -1674     
- Misses        326   2018   +1692     

[Borda]

@tchaton any update here? 🐰

[Borda]

@tchaton any update here? 🐰

[Borda]

@SkafteNicki, are we going to proceed with this one? 🐰

[SkafteNicki]

Closing in favour of #984