Fix LGTM

Signed-off-by: Beat Buesser <[email protected]>

Author: Beat Buesser

art/estimators/object_detection/tensorflow_faster_rcnn.py

@@ -100,7 +100,7 @@ def __init__(
                               `first_stage_localization_loss`, `first_stage_objectness_loss`,
                               `second_stage_localization_loss`, `second_stage_classification_loss`.
         """
-        import tensorflow as tf
+        import tensorflow as tf  # lgtm [py/repeated-import]
 
         # Super initialization
         super().__init__(