[mlir][linalg][transform] Return copy_back op from PadOp.

This patch makes the `transform.structured.pad` op return also a handle
to the copy op that it inserts. This allows to continue transformation
on that op, such as mapping it to a GPU thread.

The patch was mainly authored by @springerm as part of the WIP patch
https://reviews.llvm.org/D156371, which also has an example usage of
this change.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D159088