`clamp`: port to structured kernel (#61361)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61361
This PR ports the `clamp` kernel to the structured format. In addition, it introduces `OptionalScalarRef` as a replacement for `c10::optional<Scalar>&`. The latter, although it is a reference type, can still involve copying the contained `Scalar` (e.g. if the actual parameter is a `Scalar` or if a `c10::optional<Scalar>` is constructed just to call a kernel). `OptionalScalarRef` contains only a `const Scalar&`, and stores flag about whether the instance contains something inside the `Scalar` itself using a new tag.
For more information, see #55070.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D29821533
Pulled By: SplitInfinity
fbshipit-source-id: 88d55df5a4b2c14b68a57e4905d90eea1b088d99
