Extract version to version.txt (#27149)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27149
Extract version to version.txt and add reading version logic to setup.py and fb/torch_version.py
ghstack-source-id: 91271883
Test Plan: N/A
Reviewed By: gchanan, ezyang
Differential Revision: D17689307
fbshipit-source-id: 21899502027cec71b63d9dc151e09ff5ff3f279d
