Allow sparse matrix input to tabnet

[egillax]

Currently only dense matrices and dataframes are supported for the x matrix. This limits on how large data can be used with the current implementation. It would be good to support sparse matrices for bigger data.

[cregouby]

All internal processing of {tabnet} is done currently with dense matrix operation, so I would not see a path to use sparse matrix without a massive redesign. If you have an idea and you can propose a pull-request, I will be happy to review it.