define `get_dot_graph` (#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: https://github.com/pytorch/pytorch/pull/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
