`cuda` dependency in `quantem`

[bobleesj]

Problem

Custom CUDA written incupy enables 3-10x faster computation compared to native pytorch.

For example, using CUDA/widget, it takes about ~1s to disk-VRAM load and visualize all diffraction patterns within a jupyter notebook:

Proposed solution

Support cupy as an optional dependency.

For existing functions and files, we can stick to pytorch internal compute and numpy for user-facing APIs.

For hpc and widget modules, having cupy can be beneficial since it saves human time and enables labs with NVIDIA GPUs to utilize qunatem and their powerful hardware (a.k.a mallard ophus group)