Part 1 of implementation for general [subclass view fake-ification](https://docs.google.com/document/d/1C5taWiplmX7nKiURXDOAZG2W5VNJ2iV0fQFq92H0Cxw).
The following functional inverses are currently implemented scatter-style and thus never return views:
* `as_strided_copy_inverse()`
* `diagonal_copy_inverse()`
* `expand_copy_inverse()`
* `select_copy_int_inverse()`
* `slice_copy_Tensor_inverse()`
* `split_copy_Tensor_inverse()`
* `split_with_sizes_copy_inverse()`
* `unbind_copy_int_inverse()`
* `unfold_copy_inverse()`
We need to get actual views for the introduction of reverse view funcs coming next.
Details:
* Use `as_strided()` to implement actual view inverses for the above
* Assumes we're given a mutated_view that is actually part of a bigger storage; this isn't really the case for functionalization
* Introduce `InverseReturnMode` enum for customization of functional inverses
* `AlwaysView` - always return an actual view; needed for reverse view_funcs()
* `NeverView` - always do a copy; useful for certain functionalization use cases (e.g. XLA, executorch)
* `ViewOrScatterInverse` - return an actual view in most cases, but prefer scatter inverses when they exist. this avoids the need to implement `as_strided()` for subclasses, which can be difficult or impossible
* Make sure functionalization works as before
* Use `ViewOrScatterInverse` when reapply_views TLS is True or `NeverView` otherwise
* Adds tests to ensure old behavior for above inverses **in functionalization**
Pull Request resolved: https://github.com/pytorch/pytorch/pull/115893
Approved by: https://github.com/bdhirsh
