[dynamo] only error out on nested fx trace if dynamo is optimizing (#88640)
I think this is the final resolution to issue caused by
https://github.com/pytorch/pytorch/pull/87797. The nvfuser issue that PR
tripped up was because, even though we're correctly disabling
torchdynamo via a `DisableContext`, the nested fx trace check was still
firing. This PR properly narrows it to only fire if we're not disabled.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88640
Approved by: https://github.com/yf225
