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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (20046d4) to head (5388515).
Report is 208 commits behind head on develop.

Additional details and impacted files

@@           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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[brosaplanella]

Looks good, thanks! Is this related to the bug Jon's student found?

[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.