Add doc test in graph_drawer.py (#95919)
Add a doc test, extending #95534 .
I found I need to put the xdoctest under a class method. Otherwise if it's right under the class definition, the test cannot be found. @Erotemic Do I miss anything?
The xdoctest has been tested:
```
$ pytest --xdoctest torch/fx/passes/graph_drawer.py::FxGraphDrawer.get_dot_graph:0
=========== test session starts ==================
platform linux -- Python 3.9.15, pytest-7.2.1, pluggy-1.0.0
rootdir: /localdisk/wenzhexu/dev/forked_pytorch, configfile: pytest.ini
plugins: xdoctest-1.1.1
collected 1 item
torch/fx/passes/graph_drawer.py . [100%]
============ 1 passed in 1.13s ===================
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95919
Approved by: https://github.com/ezyang
