Remove reference to internals in torch.onnx

[justinchuby]

Describe the bug

There are reference to internal names in torch.onnx. There are not meant for external consumption and should be removed. Please comment if there are things we should expose from torch.onnx.

Urgency

When we want to support torch 1.12.

• [x] https://github.com/microsoft/onnxruntime/blob/4445dd6bc14ad6d145fa2321b4c61c84f7cd9100/onnxruntime/test/python/test_pytorch_export_contrib_ops.py#L50
• [ ] https://github.com/microsoft/onnxruntime/blob/5ecfaef042380995fb15587ccf6ff77f9d3a01d2/orttraining/orttraining/python/training/ortmodule/_custom_op_symbolic_registry.py#L9
• [ ] https://github.com/microsoft/onnxruntime/pull/11900

[jingyanwangms]

Do we need _export_onnx_opset_version, _get_tensor_dim_size, _get_tensor_sizes, parse_args from torch.onnx.symbolic_helper? Tagging people who recently changed lines with symbolic_helper methods @iK1D @garymm