Add len() for strings (#19320)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19320
ghimport-source-id: 62131cb24e9bf65f0ef3e60001cb36509a1f4163
Reviewed By: bethebunny
Differential Revision: D14961078
Pulled By: driazati
fbshipit-source-id: 08b9a4b10e4a47ea09ebf55a4743defa40c74698
