Collect all contrib models in __init__ and add unit tests for init

tests/test_contrib_model.py

@@ -0,0 +1,27 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import unittest
+
+from qlib.contrib.model import all_model_classes
+
+
+class TestAllFlow(unittest.TestCase):
+    def test_0_initialize(self):
+        num = 0
+        for model_class in all_model_classes:
+            if model_class is not None:
+                model = model_class()
+                num += 1
+        print("There are {:}/{:} valid models in total.".format(num, len(all_model_classes)))
+
+
+def suite():
+    _suite = unittest.TestSuite()
+    _suite.addTest(TestAllFlow("test_0_initialize"))
+    return _suite
+
+
+if __name__ == "__main__":
+    runner = unittest.TextTestRunner()
+    runner.run(suite())