jit technical docs - parts 1, 2, and most of 3 (#16887)
Summary:
This will evolve into complete technical docs for the jit. Posting what I have so far so people can start reading it and offering suggestions. Goto to Files Changed and click 'View File' to see markdown formatted.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16887
Differential Revision: D14191219
Pulled By: zdevito
fbshipit-source-id: 071a0e7db05e4f2eb657fbb99bcd903e4f46d84a
