define get_dot_graph (pytorch#70541)

Summary:
In the [docstring](https://github.com/pytorch/pytorch/blob/master/torch/fx/passes/graph_drawer.py#L54-L60) we mention `get_dot_graph but it is not defined, so I defined it here.
Not sure if this is preferred, or should we update the docstring to use `get_main_dot_graph`

Pull Request resolved: pytorch#70541

Test Plan:
```
            g = FxGraphDrawer(symbolic_traced, "resnet18")
            with open("a.svg", "w") as f:
                f.write(g.get_dot_graph().create_svg())
```

Reviewed By: khabinov

Differential Revision: D33378080

Pulled By: mostafaelhoushi

fbshipit-source-id: 7feea2425a12d5628ddca15beff0fe5110f4a111

torch/fx/passes/graph_drawer.py

@@ -74,6 +74,12 @@ def __init__(self, graph_module: torch.fx.GraphModule, name: str, ignore_getattr
 
                 self._dot_graphs[f"{name}_{node.target}"] = self._to_dot(leaf_node, f"{name}_{node.target}", ignore_getattr)
 
+        def get_dot_graph(self, submod_name=None) -> pydot.Dot:
+            if submod_name is None:
+                return self.get_main_dot_graph()
+            else:
+                return self.get_submod_dot_graph(submod_name)
+
         def get_main_dot_graph(self) -> pydot.Dot:
             return self._dot_graphs[self._name]