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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