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.

• onnxruntime/onnxruntime/test/python/test_pytorch_export_contrib_ops.py

  Line 50 in 4445dd6

  from torch.onnx.symbolic_helper import _export_onnx_opset_version

• onnxruntime/orttraining/orttraining/python/training/ortmodule/_custom_op_symbolic_registry.py

  Line 9 in 5ecfaef

  from torch.onnx.symbolic_helper import _get_tensor_dim_size, _get_tensor_sizes, parse_args

• Use updated symbolic_helper.check_training_mode #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