Add Selu operator support

Add Selu operator support to partially address issue:
#424

Set the version to 4 like most others.

Author: chinhuang007

tests/test_backend.py

@@ -2266,5 +2266,13 @@ def test_maxpoolwithargmax(self):
                 self.logger.debug(str(p))
                 self._run_test_case([_OUTPUT, _OUTPUT1], {_INPUT: x_val})
 
+    @check_opset_min_version(10, "Selu")
+    def test_selu(self):
+        x_val = np.random.random_sample([3]).astype(np.float32)
+        x = tf.placeholder(x_val.dtype, x_val.shape, name=_TFINPUT)
+        y = tf.nn.selu(x)
+        _ = tf.identity(y, name=_TFOUTPUT)
+        self._run_test_case([_OUTPUT], {_INPUT: x_val})
+
 if __name__ == '__main__':
     unittest_main()

tf2onnx/onnx_opset/math.py

@@ -399,3 +399,9 @@ def version_7(cls, ctx, node, **kwargs):
         ctx.remove_node(node.name)
         ctx.make_node(op_type="Sub", inputs=[node.input[0], mul.output[0]],
                       name=node.name, outputs=node.output, shapes=shapes, dtypes=dtypes)
+
+@tf_op("Selu")
+class Selu:
+    @classmethod
+    def version_4(cls, ctx, node, **kwargs):
+        pass