Add hack for legacy keras (onnx#1486)

Signed-off-by: Tom Wildenhain <tomwi@microsoft.com>

tf2onnx/convert.py

@@ -316,7 +316,20 @@ def from_keras(model, input_signature=None, opset=None, custom_ops=None, custom_
 
     # let tensorflow do the checking if model is a valid model
     function = _saving_utils.trace_model_call(model, input_signature)
-    concrete_func = function.get_concrete_function()
+    try:
+        concrete_func = function.get_concrete_function()
+    except TypeError as e:
+        # Legacy keras models don't accept the training arg tf provides so we hack around it
+        if "got an unexpected keyword argument 'training'" not in str(e):
+            raise e
+        model_call = model.call
+        def wrap_call(*args, training=False, **kwargs):
+            return model_call(*args, **kwargs)
+        model.call = wrap_call
+        function = _saving_utils.trace_model_call(model, input_signature)
+        concrete_func = function.get_concrete_function()
+        # Put it back
+        model.call = model_call
 
     # These inputs will be removed during freezing (includes resources, etc.)
     graph_captures = concrete_func.graph._captures  # pylint: disable=protected-access