Implement sparse matrices

[Markus-Goetz]

Feature functionality
Design and implement a distributed sparse DNDarray class. Come up with one or more reasonable storage formats (e.g. COO, CRS, CSC, ...)

Additional context
PyTorch implements COO only at the moment with a very restricted operation set.

Interface design might be inspired by Armadillo and/or SciPy.sprarse

[Markus-Goetz]

Here is a great paper summarizing formats and operations on sparse tensors. I would go with a COO format first where the set of entries is roughly evenly distributed

http://groups.csail.mit.edu/commit/papers/2016/parker-thesis.pdf

[ClaudiaComito]

CSR addressed in #1028 , thanks @Mystic-Slice

[ClaudiaComito]

the ht.sparse module has been implemented by @Mystic-Slice and released with v1.3.0. For the time being it supports distributed CSR matrices and element-wise operations. More work needed.

Related issues: #1039 #1040 #1046 #1043 #1044

@AmirTouilPU has initiated work on distributed COO support, PR #988 is stuck for now.

I'll close this issue for now.

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Reviewed within #1109