adds adjoint to metalens notebook

[tylerflex]

This adds a new section to the visible light metalens where we use local field projection and adjoint to fine tune the power directed to the focal spot.

We get a 17% improvement over 15 iterations.

NOTES:

• uses yaugenst-flex/polyslab-grads frontend branch, which has not been merged. without the code from this branch, the results will be terrible because the gradients are inaccurate.
• uses local gradient calculation to make use of the changes in this branch. Ideally we would switch to remote gradient.
• The gradient processing takes >5 minutes each iteration.. so I limited the code to only run one iteration. I left the cell output to correspond to the full 15 iterations that I ran locally, along with a small note.

Not sure when is the best time to merge this, maybe after Yannick's PR is merged and it is fast enough to justify running this?

[tylerflex]

@tomflexcompute this should be ready to go, I fixed the linter error and made sure it ran properly on the latest develop branch.

[github-actions]

Spell Check Report

Metalens.ipynb:

Cell 40, Line 9: 'meta-cell'
  > In this case, we use the local field projection to compute the intensity enhancement at the focal point as a function of each of the meta-cell angles.
Cell 41, Line 31: 'meta-cell'
  > """Return a simulation as a function of the meta-cell angles."""
Cell 46, Line 1: 'PuBuGn'
  > plt.imshow(grad.reshape((N, N)), cmap="PuBuGn")

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Checked 1 notebook(s). Found spelling errors in 1 file(s).
Generated by GitHub Action run: https://github.com/flexcompute/tidy3d-notebooks/actions/runs/18100448301

[tomflexcompute]

Thanks @tylerflex good to go after you correct the typos AI suggested.

[tylerflex]

Thanks @tomflexcompute , fixed all relevant typos. Apparently I have a really hard type spelling the word "function", seems like every notebook I spell that wrong.

I will merge