Generate symbol derivatives for outputs when requested

[lukasstockner]

Description

Currently, symbols are only flagged for derivative computation if

• They're a dependency of an operation which requires derivatives
• They're connected to a downstream layer parameter which requires derivatives
• They're writable globals (except for N)

However, there's a fourth relevant case - when the renderer requests the symbol to be written as an output with derivatives. This is possible via SymLocationDesc, but in the current implementation, this only ends up writing derivatives if one of the three cases above also happens to be true.

Therefore, detect this case and explicitly mark the symbol as requiring dependencies.

Tests

No tests at the moment since it would require extensive changes to testshade. Let me know if this is something that you'd consider necessary here.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format v17 before submitting, I definitely will look at
  the CI test that runs clang-format and fix anything that it highlights as
  being nonconforming.

[linux-foundation-easycla]

• ✅login: lukasstockner / (cf23166)
• ✅login: lukasstockner / (cf23166, a2cd52f)

The committers listed above are authorized under a signed CLA.

[lgritz]

CI doesn't (yet) test Windows -- I am working on it now, actually -- but I trust that you two have verified that this is the right fix, so I will merge.

[lgritz]

Sorry, that last comment was bogus, I had a different PR in mind. :-)

[lgritz]

LGTM2