Update network.py

[DanBuchan]

As of torch 2.x torch.symeigh() is no longer available. torch.linalg.eigh can be used to replace this instead

[shaunmk]

Just leaving this note here for anyone who might consider this:

This change will indeed need to be pulled if we ever intend to support newer GPUs, for example.

However, I did test torch.linalg.eigh() when the deprecation was first announced; the output coordinates are not identical to those produced when symeig() is used, and there is a small drop in model accuracy w.r.t. symeig().

Ideally we would need to retrain/finetune the model with eigh as part of the compute graph in order to do this "properly".