make gh pr descrition as part of merge pr

Author: Gasoonjia

.github/scripts/propose_ghstack_orig_pr.py

@@ -100,13 +100,16 @@ def create_prs_for_orig_branch(pr_stack: List[int], repo: Repository):
         # The PR we want to create is then "branch_to_merge" <- gh/user/x/orig
         # gh/user/x/orig is the clean diff between gh/user/x/base <- gh/user/x/head
         orig_branch_merge_head = pr.base.ref.replace("base", "orig")
+
+        try:
+            gh_pr_first_comment = pr.get_issue_comments()[0].body
+        except:
+            gh_pr_first_comment = ""
+
         bot_metadata = f"""This PR was created by the merge bot to help merge the original PR into the main branch.
 ghstack PR number: https://github.com/pytorch/executorch/pull/{pr.number} by @{pr.user.login}
+{gh_pr_first_comment}
 ^ Please use this as the source of truth for the PR details, comments, and reviews
-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}
 @diff-train-skip-merge"""
 
         existing_orig_pr = repo.get_pulls(