Fix parsing of pp split points from toml file

Previously, string_list formatting was only applied to pp split points
arg when it came from the cmd line.  The string_list formatting needs to
be applied when loading via toml too.

ghstack-source-id: fb41e66314e7fd79c02509ba68e3dc590d5f6976
Pull Request resolved: #450

torchtitan/config_manager.py

@@ -514,6 +514,15 @@ def parse_args(self, args_list: list = sys.argv[1:]):
                 logger.exception(f"Error details: {str(e)}")
                 raise e
 
+        if (
+            "experimental" in args_dict
+            and "pipeline_parallel_split_points" in args_dict["experimental"]
+        ):
+            exp = args_dict["experimental"]
+            exp["pipeline_parallel_split_points"] = string_list(
+                exp["pipeline_parallel_split_points"]
+            )
+
         # override args dict with cmd_args
         cmd_args_dict = self._args_to_two_level_dict(cmd_args)
         for section, section_args in cmd_args_dict.items():