Use `api-array-compat` as a backend

[kadykov]

What do you think about the api-array-compat project?

It should be not so difficult to change the backend from PyTorch to api-array-compat, and it will give many advantages:

• Simplify array type selection/verification logic
• Add cupy support for those who want to calculate on GPU, but don't need backpropagation or don't want to install PyTorch
• Make PyTorch dependence optional