Add string index/slice operations (#18247)
Summary:
Adds support for string indexing (`"a"[0]`) and slicing (`"abc"[1:3]`)
to script.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18247
Differential Revision: D14574486
Pulled By: driazati
fbshipit-source-id: 4b42aa0881e5398ea7f112be46c0335e6e19dced
