Remove existing causal mechanisms when creating GCM

Before, when a causal graph had causal mechanisms assigned, they were also used when creating a new GCM object based on it. Now, they are removed (from a copied version of the graph).

dowhy/gcm/causal_models.py

@@ -32,12 +32,17 @@ class ProbabilisticCausalModel:
     causal mechanisms can be any general stochastic models."""
 
     def __init__(
-        self, graph: Optional[DirectedGraph] = None, graph_copier: Callable[[DirectedGraph], DirectedGraph] = nx.DiGraph
+        self,
+        graph: Optional[DirectedGraph] = None,
+        graph_copier: Callable[[DirectedGraph], DirectedGraph] = nx.DiGraph,
+        remove_existing_mechanisms: bool = True,
     ):
         """
         :param graph: Optional graph object to be used as causal graph.
         :param graph_copier: Optional function that can copy a causal graph. Defaults to a networkx.DiGraph
                              constructor.
+        :param remove_existing_mechanisms: If True, removes existing causal mechanisms assigned to nodes if they exist.
+                                           Otherwise, does not modify graph.
         """
         # Todo: Remove after https://github.com/py-why/dowhy/pull/943.
         from dowhy.causal_graph import CausalGraph
@@ -50,6 +55,11 @@ def __init__(
         elif isinstance(graph, CausalGraph):
             graph = graph_copier(graph._graph)
 
+        if remove_existing_mechanisms:
+            for node in graph.nodes:
+                if CAUSAL_MECHANISM in graph.nodes[node]:
+                    del graph.nodes[node][CAUSAL_MECHANISM]
+
         self.graph = graph
         self.graph_copier = graph_copier