Implement DenseToSparse

[seanpmorgan]

Per the RFC, we need to move dense_to_sparse from contrib to addons:

This will involve restructuring in an OO format by inheriting from base Keras Layer, modifying the code to match those APIs, and modifying test cases to run in TF2.

Discussion: Does it make more sense to consider this a low level nn block instead of a Layer? Or just an array op?

[seanpmorgan]

tensorflow/tensorflow#23754 is a relevant discussion regarding a more efficient way to do this using cusparse

[seanpmorgan]

After some discussion we'll be closing this issue pending community outreach in support for it. It has no OSS usage as far as we can see and it may not neatly fit into our subpackages