[fx] split Node into Node/Proxy (#42991)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42991
Have Node both be a record of the operator in the graph, and the
way we _build_ the graph made it difficult to keep the IR datastructure
separate from the proxying logic in the build.
Among other issues this means that typos when using nodes would add
things to the graph:
```
for node in graph.nodes:
node.grph # does not error, returns an node.Attribute object!
```
This separates the builder into a Proxy object. Graph/Node no longer
need to understand `delegate` objects since they are now just pure IR.
This separates the `symbolic_trace` (proxy.py/symbolic_trace.py) from
the IR (node.py, graph.py).
This also allows us to add `create_arg` to the delegate object,
allowing the customization of how aggregate arguments are handled
when converting to a graph.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D23099786
Pulled By: zdevito
fbshipit-source-id: 6f207a8c237e5eb2f326b63b0d702c3ebcb254e4
