[sparse][mlir] give all sparse kernels an explicit "output" tensor

Rationale:
Providing an output tensor, even if one is not used as input to
the kernel provides the right pattern for using lingalg sparse
kernels (in contrast with reusing a tensor just to provide the shape).
This prepares proper bufferization that will follow.

Reviewed By: bixia

Differential Revision: https://reviews.llvm.org/D95587