Enable simple indexing into CSR tensor, add torch.select for CSR
This PR implements `torch.select` for CSR tensors. Currently, it's not possible to select rows or columns for batched CSR. The non-batched case works fine by converting to COO and calling select. Initially, I implemented raw manipulations of indices but converting to COO is only slightly slower and more readable.
This PR also enables indexing into batched CSR tensor with `[x, y, z]`. Assigning is disabled.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76228
Approved by: https://github.com/cpuhrsch
