Skip diff train for ghstack bot generated PR

Summary: They are landed internally. Skip the diff train for that PR.

Pull Request resolved: #6408

Differential Revision: D64708690

Author: kirklandsign

.github/scripts/propose_ghstack_orig_pr.py

@@ -100,7 +100,8 @@ def create_prs_for_orig_branch(pr_stack: List[int], repo: Repository):
 ghstack PR base: https://github.com/pytorch/executorch/tree/{pr.base.ref}
 ghstack PR head: https://github.com/pytorch/executorch/tree/{pr.head.ref}
 Merge bot PR base: https://github.com/pytorch/executorch/tree/{orig_branch_merge_base}
-Merge bot PR head: https://github.com/pytorch/executorch/tree/{orig_branch_merge_head}"""
+Merge bot PR head: https://github.com/pytorch/executorch/tree/{orig_branch_merge_head}
+@diff-train-skip-merge"""
 
         existing_orig_pr = repo.get_pulls(
             head="pytorch:" + orig_branch_merge_head,