Rename torch.namedtensor -> torch._namedtensor_internals (#26349)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26349
The directory holds a lot of private helper functions that help
implement named tensor functionality. Instead of naming each helper
function with a leading underscore, I change the name of the import to
`_namedtensor_internals` to signal it should not be used directly.
Test Plan: - [namedtensor ci]
Differential Revision: D17424178
Pulled By: zou3519
fbshipit-source-id: 8f7b74346765759303480e581038a661021acf53
