Implement Adjoint Jacobian for L2 loss function

[rht]

Before submitting

Please complete the following checklist when submitting a PR:

• [ ] All new features must include a unit test. If you've fixed a bug or added code that should be tested, add a test to the tests directory!

• [ ] All new functions and code must be clearly commented and documented. If you do make documentation changes, make sure that the docs build and render correctly by running make docs.

• [ ] Ensure that the test suite passes, by running make test.

• [ ] Add a new entry to the .github/CHANGELOG.md file, summarizing the change, and including a link back to the PR.

• [ ] Ensure that code is properly formatted by running make format.

When all the above are checked, delete everything above the dashed line and fill in the pull request template.

---

Context:

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues: