Make ONNX_ATEN_FALLBACK also works for _export (#26738)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26738
someone may use torch._export directly. Here we change the onnx_export_type's default value to None,
and if it's pytorch onnx caffe2 bundle, we set it to ONNX_ATEN_FALLBACK, otherwise, it's ONNX.
Test Plan: ci
Reviewed By: hl475
Differential Revision: D17546452
fbshipit-source-id: 38e53926e2b101484bbbce7b58ebcd6af8c42438
