change torch._dynamo.export(aten_graph=...) to allow pre_autograd tracing (#98031)
pre_autograd tracing is still early, but it should work for basic cases. This PR changes the API a bit for export to expose pre_autograd tracing. Name bikeshedding is welcome, but it looks like:
```
torch._dynamo.export(..., aten_graph="aten_pre_autograd")
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98031
Approved by: https://github.com/ezyang
