Make GHA workflow to retrieve latest promote-able SHA from master (#7…

…9548)

Relates to #76700

**Overview:** Wrote GHA to get the latest commit SHA. Another component of the script is pushing this SHA to the viable/strict branch but I'm planning to test that locally after verifying that this part is correct.

**Test Plan:** Monitor github actions results to see if the SHA printed is correct -- I wasn't able to check this on my personal fork.
Pull Request resolved: #79548
Approved by: https://github.com/seemethere

.github/workflows/update-viablestrict.yml

@@ -0,0 +1,45 @@
+name: Update viable/strict
+
+on:
+  schedule:
+    - cron: 17,47 * * * *
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
+jobs:
+  do_update_viablestrict:
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Setup Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.8
+          architecture: x64
+
+      - name: Checkout PyTorch
+        uses: pytorch/pytorch/.github/actions/checkout-pytorch@master
+
+      - name: Install Python Packages
+        run: |
+          pip3 install rockset==0.8.10
+          pip3 install boto3==1.19.12
+          pip3 install six==1.16.0
+
+      - name: Get latest viable commit
+        env:
+          ROCKSET_API_KEY: ${{ secrets.ROCKSET_API_KEY }}
+        run: |
+          output=$(python3 -m .github/scripts/print_latest_commits.py)
+          echo "::set-output name=latest_viable_sha::$output"
+        id: get-latest-commit
+
+      - name: Push SHA to viable/strict branch
+        env:
+          GITHUB_TOKEN: ${{ secrets.MERGEBOT_TOKEN }}
+        run: |
+          git config --global user.email "pytorchmergebot@users.noreply.github.com"
+          git config --global user.name "PyTorch MergeBot"
+          echo "::debug::Set the latest sha variable to be ${{ steps.get-latest-commit.outputs.latest_viable_sha }}"