[caffe2] Remove OperatorBase::newstyle_outputs_ (#67093)
`OperatorBase` maintains `output_tensors_` and `newstyle_outputs_`
which hold the same list of tensors except one is
`vector<caffe2::Tensor>` and the other is `List<at::Tensor>`.
This instead maintains only `output_tensors_` and handles the
conversions inside of export_caffe2_op_to_c10.
Differential Revision: [D32289811](https://our.internmc.facebook.com/intern/diff/D32289811)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67093
Approved by: https://github.com/dagitses, https://github.com/malfet
