This PR enables batched CSR/CSC tensors that batches may have different NSE counts.

For instance, with the current master we have
```python
>>> a = torch.tensor([[[1, 2], [3, 4]], [[0, 12], [21, 0]]])
>>> a.to_sparse_csr()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Expect the same number of specified elements per batch.
```
because the NSE of the first and second batches are different, 4 and 2, respectively.

This PR implements a strided-to-sparse-CSR/CSC conversion algorithm that supports CSR/CSC batches with different NSE counts. For instance:
```python
>>> a = torch.tensor([[[1, 2], [3, 4]], [[0, 12], [21, 0]]])
>>> b = a.to_sparse_csr()
>>> b
tensor(crow_indices=tensor([[0, 2, 4],
                            [0, 1, 2]]),
       col_indices=tensor([[0, 1, 0, 1],
                           [1, 0, 0, 0]]),
       values=tensor([[ 1,  2,  3,  4],
                      [12, 21,  0,  0]]), size=(2, 2, 2), nnz=4,
       layout=torch.sparse_csr)
>>> b[0]
tensor(crow_indices=tensor([0, 2, 4]),
       col_indices=tensor([0, 1, 0, 1]),
       values=tensor([1, 2, 3, 4]), size=(2, 2), nnz=4,
       layout=torch.sparse_csr)
>>> b[1]
tensor(crow_indices=tensor([0, 1, 2]),
       col_indices=tensor([1, 0]),
       values=tensor([12, 21]), size=(2, 2), nnz=2, layout=torch.sparse_csr)
```
that is, if the NSE of a batch is smaller than the maximum NSE over all batches, the corresponding rows in `col_indices`/`values` are padded with zeros as placeholders. Algorithms on batched CSR/CSC tensors must not access the padded parts of these tensors, that is, the algorithms should use the last element of the corresponding `crow_indices` row as the NSE value rather than the value of `.values().shape[0]` that holds the maximum NSE over all batches.

Performance-wise, the strided-to-sparse-CSR/CSC conversion algorithms in master and in this PR, are roughly equivalent:
```python
# master branch:
n [2]: a = torch.rand(10, 10, 1000, 1000)

In [3]: a = torch.where(a==0, 0.1, a)  # required for master, optional for the PR

In [4]: %timeit a.to_sparse_csr()
2.25 s ± 9.84 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [5]: a_cuda = a.cuda()

In [6]: %timeit a_cuda.to_sparse_csr()
55.2 ms ± 6.95 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
```
```python
# this PR
In [2]: a = torch.rand(10, 10, 1000, 1000)

In [3]: a = torch.where(a==0, 0.1, a)  # required for master, optional for the PR

In [4]: %timeit a.to_sparse_csr()
2.12 s ± 2.13 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [5]: a_cuda = a.cuda()

In [6]: %timeit a_cuda.to_sparse_csr(); torch.cuda.synchronize()
47.2 ms ± 10.4 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
```
The performance of `to_sparse_csr()` on CUDA tensors increased by 15% with this PR.
 
A strided-to-sparse-BSR/BSC conversion with variable NSE support will be implemented as a follow-up.




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