[mlir][sparse][taco] Support the use of index values in tensor expressions.

PyTACO DSL doesn't support the use of index values as in A[i] = B[i]+ i.
We extend the DSL to support such a use in MLIR-PyTACO.

Remove an obsolete unit test. Add unit tests and PyTACO tests.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D121716