[mlir][Tensor] Make `TilingInterface` implementation only return handle to the...

[mlir][Tensor] Make `TilingInterface` implementation only return handle to the created `pad` operation.

Pad tiling implementation only needs to return the tiled pad
operation. The rest of the generated code is related to handling
boundary conditions.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D146439