rebase bot - add ghstack config (pytorch#78561)

Fixes pytorch#78539
testing pr: https://github.com/clee2000/random-testing/pull/14
Pull Request resolved: pytorch#78561
Approved by: https://github.com/janeyx99, https://github.com/seemethere

.github/scripts/tryrebase.py

@@ -48,6 +48,14 @@ def rebase_ghstack_onto(pr: GitHubPR, repo: GitRepo, dry_run: bool = False, stab
 
     repo.fetch(orig_ref, orig_ref)
     repo._run_git("rebase", onto_branch, orig_ref)
+
+    os.environ["OAUTH_TOKEN"] = os.environ["GITHUB_TOKEN"]
+    with open('.ghstackrc', 'w+') as f:
+        f.write('[ghstack]\n' +
+                "github_url=github.com\n" +
+                "github_username=pytorchmergebot\n" +
+                "remote_name=origin")
+
     if dry_run:
         print("Don't know how to dry-run ghstack")
     else: