Revert making primary_metric w/o default type (Azure#30687)

Author: diondrapeck

sdk/ml/azure-ai-ml/azure/ai/ml/entities/_job/sweep/objective.py

@@ -27,13 +27,13 @@ class Objective(RestTranslatableMixin):
             :caption: Assigning an objective to a SweepJob
     """
 
-    def __init__(self, goal: str, primary_metric: str) -> None:
+    def __init__(self, goal: str, primary_metric: Optional[str] = None) -> None:
         """Optimization objective.
 
         :param goal: Required. Defines supported metric goals for hyperparameter tuning. Acceptable values
         are: "minimize", "maximize".
         :type goal: str
-        :param primary_metric: Required. Name of the metric to optimize.
+        :param primary_metric: Required. The name of the metric to optimize.
         :type primary_metric: str
         """
         self.goal = goal.lower()