fix[invdes]: handle pixel size validation for simulations without sources

[yaugenst-flex]

Currently InverseDesign validation will error if the simulation contains no sources due to the _check_pixel_size validator trying to access sim.wvl_mat_min, which depends on the sources being present.
This PR skips the validator if the simulation contains no sources and instead warns the user about checking the pixel size. The reason for not erroring is that we probably still want to allow the construction of simulations without sources in the plugin since we also allow it in td.Simulation (and it can be useful in some cases).

[tylerflex]

Porting from other discussion: do we want to just fix sim.wvl_mat_min to use some default value even if sources are not defined? I think there was a related discussion about the RunTimeSpec not working with no sources present

[yaugenst-flex]

@momchil-flex @tylerflex I think it might be best to raise an error in sim.wvl_mat_min in case sources are not present. Returning an arbitrary value seems incorrect. This fix would also work for the invdes plugin since the error message is propagated. So something along the lines of Simulation contains no sources, cannot determine wavelength in material would work fine.

[notion-workspace]

Better validation error if no source in simulation for InverseDesign

[e-g-melo]

Thanks @yaugenst-flex!

I just gave it a try, and it works well. In which cases do you plan to run optimizations without sources? Eigenmode optimizations?

Emerson

[yaugenst-flex]

I just gave it a try, and it works well. In which cases do you plan to run optimizations without sources? Eigenmode optimizations?

Great! Actually it's not as much about running the optimization but just being able to set up optimizations, because a common pattern we use involves doing base_something.updated_copy(field=new_items). It can also be useful when running locally to test the penalties maybe. But if we end up going for the sim.wvl_mat_min error then we'll error instead, which should also be fine. In that case we can give a good error message that there are no sources present, which should also be fine for GUI right?

[e-g-melo]

Great! Actually it's not as much about running the optimization but just being able to set up optimizations, because a common pattern we use involves doing base_something.updated_copy(field=new_items). It can also be useful when running locally to test the penalties maybe. But if we end up going for the sim.wvl_mat_min error then we'll error instead, which should also be fine. In that case we can give a good error message that there are no sources present, which should also be fine for GUI right?

Got it!

Yes, it should also work for GUI.

[yaugenst-flex]

Merging now but will follow up with a fix for sim.wvl_mat_min