Conda build (#38796)
Summary:
closes gh-37584. ~I think I need to do more to generate an image, but the `.circleci/README.md` is vague in the details. The first commit reflows and updates that document a bit, I will continue to update it as the PR progresses :)~ Dropped updating `.circleci/README.md`, will do that in a separate PR once this is merged.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38796
Reviewed By: gchanan
Differential Revision: D22627522
Pulled By: ezyang
fbshipit-source-id: 99d5c19e942f15b9fc10f0de425790474a4242ab
