Automatic testing of examples

The CircleCI test should fail.

tests/test_examples.py

+# coding=utf-8
+# Copyright 2019-present, the HuggingFace Inc. team.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+import os
+
+
+def get_examples_from_file(file):
+    examples = []
+    example = []
+    example_mode = False
+    example_indentation = None
+    for i, line in enumerate(file):
+        if example_mode:
+            current_indentation = len(line) - len(line.strip()) - 1
+            if current_indentation == example_indentation or '"""' in line:
+                example_mode = False
+                example_indentation = None
+                examples.append(example)
+                example = []
+            else:
+                if line is not "\n":
+                    example.append(line[example_indentation + 4 : -1])
+        if "example::" in line.lower():
+            example_mode = True
+            example_indentation = line.lower().find("example::")
+
+    return examples
+
+
+class TestCodeExamples(unittest.TestCase):
+    def test_configuration_examples(self):
+        transformers_directory = "../src/transformers"
+        configuration_files = [file for file in os.listdir(transformers_directory) if "configuration" in file]
+
+        for configuration_file in configuration_files:
+            with open(os.path.join(transformers_directory, configuration_file)) as f:
+                examples = get_examples_from_file(f)
+                print("Testing", configuration_file, str(len(examples)) + "/" + str(len(examples)))
+
+                def execute_example(code_example):
+                    for line in code_example:
+                        exec(line)
+
+                with self.subTest(msg=configuration_file):
+                    [execute_example(code_example) for code_example in examples]