refactor(notebooks): use internal adam and optimize helper - batch 4

[marcorudolphflex]

follow-up from #463

Replaces the optax adam with our adam and make usage of the optimize helper when easy possible. This is the fourth batch of it with 5 out of ~20 notebooks to make that reviewable.

results are roughly unchanged, here some notable differences: (will insert images here later)

Autograd25WaveguideCrossing.ipynb: had to change fwidth from f0/20 to f0/5, got glitchy transmissions > 1 otherwise.
Objective (max.): develop 9.6852e-01 vs. branch 9.4947e-01
develop:

branch:

Autograd26DiffractiveBeamSplitter.ipynb
develop:

Loss value (maximize): 0.8790
  Cost (minimize): 9.2712e-01
  Fab penalty: 9.9816e-01
  Efficiency: 0.9999

branch:

Loss value (maximize): 0.8792
  Cost (minimize): 9.2696e-01
  Fab penalty: 9.9816e-01
  Efficiency: 0.9999

Autograd27Smatrix.ipynb
Objective (min.): develop 1.0909e-02 vs branch 3.6146e-02 ⚠️
develop

branch

Autograd28FiberLens.ipynb
objective (max.): develop 4.0160e-02 vs. branch 4.0133e-02
(plot/result nearly unchanged)

Autograd7Metalens.ipynb
objective (max.): develop 2.2378e+01 vs. branch 2.3344e+01
(plot/result nearly unchanged)

[github-actions]

Spell check passed successfully for 5 notebook(s).
Generated by GitHub Action run: https://github.com/flexcompute/tidy3d-notebooks/actions/runs/23840893792

[marcorudolphflex]

will rerun Autograd27 with other seed to check if we can still see that objective setback.