Setting fallback=True in torch.onnx.export complicates the error messages for users using the exporter with default settings, who are expecting the exporter to take the dynamo path. It will tall users to provide the f arg, for example, and flushes the dynamo error messages with torchscript errors if both paths fails etc.

Overall, it leads to a confusing experience and IMO should only be turned on at user request.

cc @titaiwangms @xadupre