docs: add code snippets for imported onnx tutorials

[ashleyxuu]

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  The tutorial is for page: https://cloud.google.com/bigquery/docs/making-predictions-with-sklearn-models-in-onnx-format

Fixes internal #338841583 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/imported_onnx_model_test.py:23, tag bigquery_dataframes_imported_sklearn_onnx_tutorial_import_tensorflow_models
• samples/snippets/imported_onnx_model_test.py:37, tag bigquery_dataframes_imported_sklearn_onnx_tutorial_make_predictions

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