FEAT: allow offloading/reloading large tensors to/from CPU

[mdw771]

Features/fixes

Added PtychographyTask.set_large_tensor_device, which moves following data between GPU and CPU:

• object buffer
• probe buffer
• forward model intermediate variables
• diffraction patterns

Usage:

# Offloading tensors to CPU
task.set_large_tensor_device("cpu")

# Reloading tensors to GPU
task.set_large_tensor_device("cuda")

Related issues (optional)

#52

Mentions

@yijiang1

Checklist

Have you...

• Formatted your code properly adhering to PEP-8? Considering using RUFF to format your code automatically.
• Resolved all merge conflicts with the main branch?
• Checked the diffs between your branch and the main branch to ensure that your changes are not introducing any regressions or unnecessary/irrelevant changes?