(torchx/cli)(bugfix) gracefully handle leading -- in torchx run, when no component args are present

Summary:
If user specifies BOTH:
1. the default component name
2. ALL default component args

In `.torchxconfig`, then
```
$ torchx run
$ torchx run --
```
should both work, but the second case is not handled properly due to a missing length check after removing the leading "--".

Reviewed By: d4l3k

Differential Revision: D33641288

fbshipit-source-id: a4089704dd67206f8f5b3a959e0e56f2f39d506c

Author: Kiuk Chung

torchx/cli/cmd_run.py

@@ -102,6 +102,7 @@ def _parse_component_name_and_args(
         if args[0] == "--":
             args = args[1:]
 
+    if len(args) > 0:  # check len again in case we removed the leading "--" above
         if args[0].startswith("-"):
             component_args = args
         else:  # first element is NOT an option; then it must be a component name

torchx/cli/test/cmd_run_test.py

@@ -219,6 +219,9 @@ def test_parse_component_name_and_args_no_default(self) -> None:
             _parse_component_name_and_args(["utils.echo", "--msg", "hello"], sp),
         )
 
+        with self.assertRaises(SystemExit):
+            _parse_component_name_and_args(["--"], sp)
+
         with self.assertRaises(SystemExit):
             _parse_component_name_and_args(["--msg", "hello"], sp)
 
@@ -248,6 +251,10 @@ def test_parse_component_name_and_args_with_default(self) -> None:
             ("custom.echo", []),
             _parse_component_name_and_args([], sp, dirs),
         )
+        self.assertEqual(
+            ("custom.echo", []),
+            _parse_component_name_and_args(["--"], sp, dirs),
+        )
         self.assertEqual(
             ("custom.echo", ["--msg", "hello"]),
             _parse_component_name_and_args(["--", "--msg", "hello"], sp, dirs),