[package] Add docstring for PackageExporter.intern (#59602)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59602
**Summary**
This commit adds a docstring for `PackageExporter.intern`.
**Test Plan**
Continuous integration.
Test Plan: Imported from OSS
Reviewed By: jbschlosser
Differential Revision: D28972939
Pulled By: SplitInfinity
fbshipit-source-id: 1765541aa2ed88e01beb48c08b90f56df3a591b7
