[mlir][sparse][taco] Add support for scalar tensors.

This change allows the use of scalar tensors with index 0 in tensor index
expressions. In this case, the scalar value is broadcast to match the
dimensions of other tensors in the same expression.

Using scalar tensors as a destination in tensor index expressions is not
supported in the PyTACO DSL.

Add a PyTACO test to show the use of scalar tensors.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D120524