Fix namedtensor ci (#26257)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26257
In native_functions.yaml, all overloads must have unique overload names.
This PR fixes `flatten` to have unique names for the overloads.
Test Plan: - tested locally, but also [namedtensor ci]
Differential Revision: D17391243
Pulled By: zou3519
fbshipit-source-id: aaef654953b4275c43b9d7bd949c46bd011f6c73
