[fx] use a linked list for nodes (#45708)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45708
This makes it possible to define reasonable semantics for what happens
when a node in the list is deleted. In particular the iteration over nodes
will continue at the node that was after the deleted node _when it was deleted_.
If the new node is also deleted, we skip it and, continue to the node after it.
Eventually we either reach a node still in the list or we reach the end of the list.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D24089516
Pulled By: zdevito
fbshipit-source-id: d01312d11fe381c8d910a83a08582a2219f47dda
