Add sum to _sparse_py_operators

[aerubanov]

When working on pymc-devs/pymc#4596, I needed the sum() method for sparse matrices for doing things like these:

import numpy as np, aesara as as

 W = np.array(
            [[0.0, 1.0, 1.0, 0.0], [1.0, 0.0, 0.0, 1.0], [1.0, 0.0, 0.0, 1.0], [0.0, 1.0, 1.0, 0.0]]
        )
W = as.tensor.as_tensor_variable(W)
W = as.sparse.csr_from_dense(W)
D = W.sum(axis=0)

and as a result, D should be dense vector [2.0, 2.0, 2.0, 2.0].

[twiecki]

@aerubanov Can you add more complete example code that imports modules and creates a sparse matrix?

[brandonwillard]

There is already support for sums on sparse matrices; see here. If you're having specific issues with this functionality, you can open another issue or update this one with a minimum working example; otherwise, I'll assume this particular issue is only about adding a sum method.

[aerubanov]

@brandonwillard Thank you, looks like it is exactly what I need! I just couldn't find it in the documentation. So I close the issue.

[brandonwillard]

We should still have a sum method, so I'm reopening it for that.

[aerubanov]

This feature was implemented in #745, so I close issue.