Amend commit to gh-pages branch (#3345)

Summary:
This commit changes the way doc is pushed.
It ammends instead of adding a new commit.

Currently each commit in gh-pages contain like 100MB of data. gh-pages branch is fetched by default when `git clone`. So the size of torchaudio repo grows significantly.

Pull Request resolved: https://github.com/pytorch/audio/pull/3345

Reviewed By: nateanl

Differential Revision: D46136612

Pulled By: mthrok

fbshipit-source-id: 39479ee5d1a6888254ef50f0db252453d976d183

.github/workflows/build_docs.yml

@@ -142,6 +142,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         ref: gh-pages
+        fetch-depth: 5
     - uses: actions/download-artifact@v3
       with:
         name: docs
@@ -149,13 +150,20 @@ jobs:
       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 +%u" = "1" ]; then
+           git commit --allow-empty -m "placeholder"
+        fi
+
         # TODO: add tag-based process (need to handle the main directory name)
+        # Update the main doc
         rm -rf main
         mv html main
-
-        # Update the main doc
         git add --all main || true
-        git config user.name "pytorchbot"
-        git config user.email "soumith+bot@pytorch.org"
-        git commit -m "auto-generating sphinx docs" || true
-        git push
+
+        git commit --amend -m "auto-generating sphinx docs" || true
+        git push -f