Update symbolics policy to emit aten::ATen for Caffe2 build only
Currently ONNX exporter symbolics can emit ATen operators when `operator_export_type==ONNX_ATEN_FALLBACK`. However, this is a behavior specific to Caffe2 builds, as the intend use of `ONNX_ATEN_FALLBACK` is to emit ATen operators only when there is no ONNX equivalent.
The reason Caffe2 choses to emit ATen operators when ONNX counterpart exists is for performance on their particular engine implementation, which might not be true for other implementations. e.g. ONNX Runtime can optimize the generated ONNX graph into something more efficient
This PR must be merged only after https://github.com/pytorch/pytorch/pull/73954
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74680
Approved by: https://github.com/garymm, https://github.com/malfet
