Remove torch_to_cupy and cupy_to_torch functions

[aknierim]

Cupy's fromDlpack and toDlpack as well as PyTorch's torch.utils.dlpack.to_dlpack methods are deprecated.

Packages following the array API standard like Cupy and PyTorch now provide the __dlpack__ method which allows conversion through the array_namespace.from_dlpack method.

import torch
import cupy as cp

test_tensor = torch.tensor([1])
print("__dlpack__" in dir(test_tensor))  # True

test_array = cp.array([1])
print("__dlpack__" in dir(test_array))  # True