[mlir][sparse][gpu] a first prototype sparse GPU code generator

This implements a proof-of-concept GPU code generator
to the sparse compiler pipeline, currently only capable
of generating CUDA threads for outermost parallel loops.

The objective, obviously, is to grow this concept
to a full blown GPU code generator, capable of the
right combinaton of code generation as well as exploiting
idiomatic kernels or vector specific libraries (think cuSparse).

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D147483