[mlir][sparse] Refactoring: abstract sparse tensor memory scheme into a...

[mlir][sparse] Refactoring: abstract sparse tensor memory scheme into a SparseTensorDescriptor class.

This patch abstracts sparse tensor memory scheme into a SparseTensorDescriptor class. Previously, the field accesses are performed in a relatively error-prone way, this patch hides the hairy details behind a SparseTensorDescriptor class to allow users access sparse tensor fields in a more cohesive way.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D138627