Fix sym_{sizes,strides} slow path (#107839)
Previously, when SymInt is returned from sym_sizes slow path, it would segfault.
This is useful for tensors that have symbolic sizes and use the sym_sizes slow path, e.g. NestedTensor returning SingletonSymInt as its sizes in the slow path.
See also: https://github.com/pytorch/pytorch/pull/106405/files#r1303714865
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107839
Approved by: https://github.com/ezyang
