move secondary broadcast outside of gradient

[valentinsulzer]

Description

This fixes the edge case for #2837 / removes the need to use x-average

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)

Key checklist:

• [ ] No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• [ ] All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• [ ] The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added that prove fix is effective or that feature works

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.58%. Comparing base (20046d4) to head (5388515). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #4190 +/- ## ======================================== Coverage 99.58% 99.58% ======================================== Files 288 288 Lines 21797 21799 +2 ======================================== + Hits 21706 21708 +2 Misses 91 91 ```

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[valentinsulzer]

I haven't followed that bug, what is it?

[brosaplanella]

I believe it was an issue with pybamm.grad(2*c). Having a 1 worked but having a 2 crashed it. I think Rob knows more about it.