clean up NamedTuple creation API (#28189)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28189
This makes it a separate createNamed function. The existing API resulted
in poor usage in fbcode, which in turn caused bugs in TorchScript programs.
Test Plan: Imported from OSS
Differential Revision: D17970220
Pulled By: zdevito
fbshipit-source-id: 59b082a726f56bec1c8d10d410db829f4aa271ea
