More autograd improvements

[tylerflex]

Introduces a set of additional improvements to autograd plugin:

1. PolySlab grad monitor data is only stored on the plane where needed.
2. Validate FWD sim, even when it's a server-side run.
3. Properly handle error if no adjoint sources are present and add a more useful fix.

Let me know if you'd like this merged int #1891 for one PR? But would be good to include both in a 2.7.3

[yaugenst-flex]

LGTM

(also, I'm in favor of small PR)