Change default value of layout in view_model of CausalModel

Changing the value from "dot" to None. The current default value requires the dot extension to be installed. However, if it is not installed, an error is raised. By default, this should not happen since it is an optional dependency.

Signed-off-by: Patrick Bloebaum <bloebp@amazon.com>

dowhy/causal_model.py

@@ -434,7 +434,7 @@ def refute_estimate(self, estimand, estimate, method_name=None, show_progress_ba
         res = refuter.refute_estimate(show_progress_bar)
         return res
 
-    def view_model(self, layout="dot", size=(8, 6), file_name="causal_model"):
+    def view_model(self, layout=None, size=(8, 6), file_name="causal_model"):
         """View the causal DAG.
 
         :param layout: string specifying the layout of the graph.