Fix missing move and bare new in pytree from_str_internal (#6803)

* Use std::variant to implement pytree Key

Pull Request resolved: https://github.com/pytorch/executorch/pull/6701

Key was a struct that should've been a union; std::variant makes using a union much easier.
ghstack-source-id: 253128071
@exported-using-ghexport

Differential Revision: [D65575184](https://our.internmc.facebook.com/intern/diff/D65575184/)

* Fix missing move and bare new in pytree from_str_internal

Just a couple minor fixes.

Differential Revision: [D65576543](https://our.internmc.facebook.com/intern/diff/D65576543/)

ghstack-source-id: 253128072
Pull Request resolved: https://github.com/pytorch/executorch/pull/6783

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Co-authored-by: Scott Wolchok <swolchok@fb.com>