Fix coverage in generation_strategy.py

Summary: As titled

Reviewed By: bernardbeckerman

Differential Revision: D41088315

fbshipit-source-id: 93f513f733c67e165d6dd5f59b8db79f827c509e

ax/modelbridge/generation_strategy.py

@@ -601,7 +601,7 @@ def _maybe_move_to_next_step(self, raise_data_required_error: bool = True) -> bo
                 ]
             ):
                 if len(self._steps) == self._curr.index + 1:
-                    raise GenerationStrategyCompleted(
+                    raise GenerationStrategyCompleted(  # pragma: no cover
                         f"Generation strategy {self} generated all the trials as "
                         "specified in its steps."
                     )