Integrating complex tensors

[flaport]

As long as PyTorch does not sufficiently support complex numbers out of the box, complex number support won't be added to Photontorch. This issue is here to track the progress of Pytorch in integrating complex tensors.

As I understand now, PyTorch complex tensors are sufficiently integrated to perform the simulation part of Photontorch, but lack sufficient autograd support to allow for optimization with backpropagation.