Add support for Keras resnet50 pretrained model testing.

Author: jignparm

ci_build/azure_pipelines/pretrained_model_test.yml

@@ -20,7 +20,7 @@ jobs:
 - template: 'templates/job_generator.yml'
   parameters:
     platforms: ['windows']
-    tf_versions: ['1.14.0', '2.1.0']
+    tf_versions: ['1.14.0']
     job:
       steps:
       - template: 'pretrained_model_test.yml'

tf2onnx/tf_loader.py

@@ -260,6 +260,7 @@ def from_keras(model_path, input_names, output_names):
     # Handles Keras when Eager mode is enabled.
     custom_objects = None
     if context.executing_eagerly():
+        _keras.backend.clear_session()
         _keras.backend.set_learning_phase(False)
         keras_model = _keras.models.load_model(model_path, custom_objects)