[dynamo] Error when user nests FX with dynamo (#87797)
Today, this doesn't work and dynamo errors out in a very non-obvious way (see:
https://gist.github.com/suo/dde04830372ab51a4a34ea760f14200a).
Here, we detect the error early and exit with a nicer msg. Also add a
config option to just no-op dynamo (which need to unblock internal
enablement).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87797
Approved by: https://github.com/yf225, https://github.com/soumith, https://github.com/jansel
