fix field data flux

[tylerflex]

The adjoint plugin FieldData.flux previously returned a float that summed over all of the flux values.

For multi-frequency adjoint, this is not quite what we want, rather we want a JaxDataArray with frequency coordinate.

(I think this was added before multi-frequency adjoint and just wasn't fixed when that was introduced).

This PR properly returns a JaxDataArray (FluxDataArray).

A potential issue for backwards compatibility if users have .flux in their notebooks. I'm not sure how to resolve this. Options are

1. Just change it to the "expected" behavior (return data array) and explain if users are confused.
2. Do some automatic sum() if certain conditions are met (like a single frequency coordinate)
3. Add a sum: bool=True kwarg so we can instruct users that want all the flux values to set this to False

fyi @tomflexcompute

[momchil-flex]

Ah. Kinda bad. I kinda prefer 1 (not sure how 2 differs, if this is an issue only with multi-freq). It's probably rare to want to sum the values, and even if you do, you can still do it very easily in your own function. Not sure how many people we'll have to explain this to but it feels like we should just pull the band-aid...

[tylerflex]

Option 2 would keep the current handling (return float) for single frequency, but return a DataArray for multi-frequency.

[tylerflex]

I just revised this PR to try the handling in 2. see the new code here:

tidy3d/tidy3d/plugins/adjoint/components/data/monitor_data.py

Lines 163 to 174 in 1b39cc2

	def package_flux_results(self, flux_values: JaxDataArray) -> Union[float, JaxDataArray]:
	"""How to package the dictionary of fields computed via self.colocate()."""
	freqs = flux_values.coords.get("f")
	# handle single frequency case separately for backwards compatibility
	# return a float of the only value
	if freqs is not None and len(freqs) == 1:
	return jnp.sum(flux_values.values)
	# for multi-frequency, return a JaxDataArray
	return flux_values

also added a more proper test of both cases.

Maybe I'd prefer this for now?

[momchil-flex]

I think this makes sense. Do we really want to put it in a patch though? Since we're planning the 2.7.0rc1 for Monday is it better to just put it there?

[tylerflex]

Fine with me. I'll try to change it to pre/2.7 before Monday

[tylerflex]

@momchil-flex rebased against 2.7