Fix docs build workflow permission (#1639)

Summary:
As reported by henrylhtsang , the docs build workflow has been failing since we downgrade the default GITHUB_TOKEN permission to read-only org-wide.  Write permission needs to be granted explicitly by the workflow now.

### Testing

Run the workflow manually with my branch https://github.com/pytorch/torchrec/actions/runs/7574439344

Pull Request resolved: #1639

Reviewed By: henrylhtsang

Differential Revision: D52878961

Pulled By: huydhn

fbshipit-source-id: fbdc96208f7b4788e1abd01071aa6d331095ec1e

.github/workflows/docs.yml

@@ -8,6 +8,9 @@ on:
 jobs:
   build_docs_job:
     runs-on: ${{ matrix.os }}
+    permissions:
+      # Grant write permission here so that the doc can be pushed to gh-pages branch
+      contents: write
     strategy:
       matrix:
         include: