Excise dependency on networkx

[zou3519]

We want to merge functorch's build system into pytorch so we can package functorch and pytorch together. In order to do that, we want to make sure we don't take additional dependencies on other projects to preserve pytorch's portability.

The time has come to revisit our networkx dependency. cc @Chillee

[vfdev-5]

This can be optional as tensorboard for pytorch utils https://pytorch.org/docs/stable/tensorboard.html?highlight=tensorboard#module-torch.utils.tensorboard

[zou3519]

Optional should be fine for now