Port smooth_l1_loss to structured kernels (#67404)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67404
Port smooth_l1_loss to structured kernels.
Brian Hirsh authored the part of adding build_borrowing_binary_op_coerce_to_scalar to TensorIterator.
Test Plan: This commit shouldn't change the behavior. So, CI.
Reviewed By: bdhirsh, ngimel
Differential Revision: D31981147
Pulled By: alanwaketan
fbshipit-source-id: a779bb76c848eed8b725dc0e1d56b97a3bd9c158
