Add missing UV partials computation for duv_dy AOV type

[winbergs]

Description

The UV partials were computed for AOVType::dUVdx, but missing in the AOVType::dUVdy case in aov.cpp. This PR adds the missing si.compute_uv_partials(ray) statement also for the duv_dy case.

Checklist

• My code follows the style guidelines of this project
• My changes generate no new warnings
• My code also compiles for cuda_* and llvm_* variants. If you can't test this, please leave below
• I have commented my code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I cleaned the commit history and removed any "Merge" commits
• I give permission that the Mitsuba 3 project may redistribute my contributions under the terms of its license