Fix a bug in TorchBench ondemand CI. (pytorch#67743)

Summary:
Use the main branch when TorchBench branch is not specified.

RUN_TORCHBENCH: soft_actor_critic

Pull Request resolved: pytorch#67743

Reviewed By: seemethere

Differential Revision: D32142663

Pulled By: xuzhao9

fbshipit-source-id: 160227835543b8e55c970025073839bf0f03aa81

.github/scripts/run_torchbench.py

@@ -74,7 +74,7 @@ def extract_models_from_pr(torchbench_path: str, prbody_file: str) -> List[str]:
     return model_list
 
 def identify_torchbench_branch(torchbench_path: str, prbody_file: str) -> None:
-    branch_name: str
+    branch_name: str = ""
     with open(prbody_file, "r") as pf:
         lines = map(lambda x: x.strip(), pf.read().splitlines())
         magic_lines = list(filter(lambda x: x.startswith(MAGIC_TORCHBENCH_PREFIX), lines))