Fix commit races on binary CI on master PR-merges (#16773)
Summary:
There is no way to test this until it is merged.
On master jobs that run after a PR is merged, there is no CIRCLE_PR_NUMBER so the binary builds clone pytorch/pytorch/master, which races.
Based off of https://circleci.com/docs/2.0/env-vars/ and the circleci checkout code
```
git config --global url."ssh://git@github.com".insteadOf "https://github.com" || true
git config --global gc.auto 0 || true
if [ -e /home/circleci/project/.git ]
then
cd /home/circleci/project
git remote set-url origin "$CIRCLE_REPOSITORY_URL" || true
else
mkdir -p /home/circleci/project
cd /home/circleci/project
git clone "$CIRCLE_REPOSITORY_URL" .
fi
if [ -n "$CIRCLE_TAG" ]
then
git fetch --force origin "refs/tags/${CIRCLE_TAG}"
else
git fetch --force origin "master:remotes/origin/master"
fi
if [ -n "$CIRCLE_TAG" ]
then
git reset --hard "$CIRCLE_SHA1"
git checkout -q "$CIRCLE_TAG"
elif [ -n "$CIRCLE_BRANCH" ]
then
git reset --hard "$CIRCLE_SHA1"
git checkout -q -B "$CIRCLE_BRANCH"
fi
git reset --hard "$CIRCLE_SHA1"
```
I believe we do no use git tags
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16773
Differential Revision: D13962132
Pulled By: pjh5
fbshipit-source-id: c62d2139f38ff39ecda1509b0bcd8bd102828e40
