Orca tf save_keras_model add overwrite option (intel-analytics#2835)

* orca tf save_keras_model add overwrite option

* add default value & update docs

* fix

python/orca/src/bigdl/orca/learn/tf/estimator.py

@@ -183,10 +183,11 @@ def save_tf_checkpoint(self, path):
         """
         raise NotImplementedError()
 
-    def save_keras_model(self, path):
+    def save_keras_model(self, path, overwrite=True):
         """
         Save tensorflow keras model in this estimator.
         :param path: keras model save path.
+        :param overwrite: Whether to silently overwrite any existing file at the target location.
         """
         raise NotImplementedError()
 
@@ -576,5 +577,5 @@ def evaluate(self, data, batch_size=4,
 
         return self.model.evaluate(dataset, batch_per_thread=batch_size)
 
-    def save_keras_model(self, path):
-        self.model.save_model(path)
+    def save_keras_model(self, path, overwrite=True):
+        self.model.save_model(path, overwrite=overwrite)