[RFC] Create APIs for pytorch

[justinchuby]

PyTorch calls onnxscript apis and replies on stability of the apis because a single pytorch version can stay for a long time. I propose that we create apis dedicated to a specific version of PyTorch like below. As we evolve our logic we are more free to change user apis like optimize without being constrained by existing PyTorch usage. This also allows use to evolve the logic for these apis even after a PyTorch version is released.

onnxscript._framework_apis.torch_2_5.optimize()
onnxscript._framework_apis.torch_2_5.convert_version()
onnxscript._framework_apis.torch_2_5.check_model()
onnxscript._framework_apis.torch_2_5.offload_external_data()
onnxscript._framework_apis.torch_2_5.get_torchlib_ops()

cc @gramalingam @xadupre @titaiwangms @shubhambhokare1

[titaiwangms]

Are these the only 4 apis? I think I only recognize optimize(). I think as long as ONNX IR doesn't have PyTorch version issue, this is fine.

[justinchuby]

Added get_torchlib_ops()