Continue cleaning up the sparse/dense tensor types

[brandonwillard]

Currently, TensorTypes aren't an abstract class, and direct instances of TensorTypes represent dense tensors, as do instances of the new DenseTensorType.

Instead, it would be better to have and use a single abstract tensor Type and distinct sparse and dense tensor Types. In this case, we could remove all the meta classes.

One of the issues I can see with this change: external libraries that use TensorType and/or TensorVariables directly will need to change these to DenseTensorType and DenseTensorVariable, unless we continue to keep these names reserved for dense tensors and use new AbstractTensor[Type|Variable] classes as the shared base.

Originally posted by @brandonwillard in #766 (review)

[ricardoV94]

Sounds less painful to keep the current TensorType as is and add the new abstract class

[rlouf]

Sounds less painful to keep the current TensorType as is and add the new abstract class

I agree, and although DenseTensorType is more explicit I think it would confuse most people. Everyone is used to "dense" being the default.

[brandonwillard]

Yeah, we can make Tensor[Type|Variable] aliases for DenseTensor[Type|Variable] and remove all uses of Tensor[Type|Variable] from internal code. That should clear things up a lot.

[danhphan]

Hi, I can work on this task. Thanks

[brandonwillard]

Hi, I can work on this task. Thanks

That would be great!