Add typing_extensions to requirements.txt (#1450)

Summary:
Pull Request resolved: #1450

We use `typing_extensions` in a few places, but it wasn't listed on our dependencies.
Add it to ensure it's installed.
4.12 is the last version with a change to an API we used, so anything newer than that is
fine.

Reviewed By: benjipelletier

Differential Revision: D84078256

fbshipit-source-id: b075611b38da5667ba3b3bc6963097816e4e2b84

Author: dulinriley

requirements.txt

@@ -4,6 +4,7 @@ pyzmq
 requests
 numpy
 pyre-extensions
+typing-extensions>=4.12
 cloudpickle
 torchx-nightly
 lark