Add automated doc flow for release (#3634)

Currently, nightly documentation is pushed automatically for each commit and on daily basis.

For release, automating it is tricky because we need to ensure that the version number is properly updated.
(removing alpha suffix at branch cut time and updating minor version at minor release)

For this reason, in release/2.1 we used manual-trigger to build and deploy doc for release.
This commit back port it to main.

For each release, once the version number is updated, running build_doc job from GHA will deploy the documentation automatically.

.github/workflows/build_docs.yml

@@ -130,3 +130,45 @@ jobs:
 
         git commit --amend -m "auto-generating sphinx docs" || true
         git push -f
+
+  # Push for release
+  # Make sure that version.txt is updated first (alpha suffix is removed)
+  commit-release:
+    if: startsWith(github.ref_name, 'release/') && ( github.event_name == 'workflow_dispatch' )
+    permissions:
+      # Required for `git push`
+      # Note:
+      #   This is not effective from fork.
+      #   When you debug this, make sure to make a branch on pytorch and
+      #   make PR from there.
+      contents: write
+    runs-on: ubuntu-latest
+    needs: build
+    steps:
+    - uses: actions/checkout@v3
+      with:
+        ref: gh-pages
+        fetch-depth: 5
+    - uses: actions/download-artifact@v3
+      with:
+        name: docs
+    - name: Update doc
+      run: |
+        set -x
+
+        git config user.name "pytorchbot"
+        git config user.email "soumith+bot@pytorch.org"
+        # When `git clone`, `gh-pages` branch is fetched by default.
+        # The size of gh-pages grows significantly, so we use ammend and force push
+        # We add a new commit once a week
+        if [ "$(date +%d)" = "1" ]; then
+           git commit --allow-empty -m "placeholder"
+        fi
+
+        dirname="$(cat version.txt)"
+        rm -rf "${dirname}"
+        mv html "${dirname}"
+        git add --all "${dirname}" || true
+
+        git commit --amend -m "auto-generating sphinx docs" || true
+        git push -f