[PyTorch] Move get_nested_tensor_impl to header (#73998)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73998
For reuse elsewhere.
ghstack-source-id: 150983356
Test Plan: CI
Reviewed By: ezyang
Differential Revision: D34647899
fbshipit-source-id: 0a11859bfaaa84f095e1fd10244a6a58c4071608
(cherry picked from commit de5026915fdc80cd704e7783b63e859e2cf045df)
