[Documentation] Python tutorial missing PyTorch export guidance and external data file handling

### Describe the documentation issue

The Python inference tutorial (https://onnxruntime.ai/docs/api/python/tutorial.html) only covers sklearn → ONNX. The PyTorch export tutorial
(https://onnxruntime.ai/docs/tutorials/export-pytorch-model.html) only covers custom operators.

Neither page documents a common production gotcha: exporting large PyTorch models with opset 17+ automatically produces two files (model.onnx + model.onnx.data). InferenceSession fails if only model.onnx is present without the companion .data file.

Also undocumented: how to merge the split files into a single, self-contained .onnx file for distribution/deployment.

### Page / URL

https://onnxruntime.ai/docs/tutorials/export-pytorch-model.html

Start with the export-pytorch-model.html page and the Python inference tutorial linked in the issue. Verify the documented behavior for large PyTorch models with opset 17+ and external model data, then explain the companion .onnx.data requirement and how to merge the split files for a self-contained deployment file.

Written by the indexing model from the issue text.