docs: add code snippets for imported tensorflow model

[ashleyxuu]

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Fixes internal #338878382 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/imported_tensorflow_model_test.py:23, tag bigquery_dataframes_imported_tensorflow_tutorial_import_tensorflow_models
• samples/snippets/imported_tensorflow_model_test.py:37, tag bigquery_dataframes_imported_tensorflow_tutorial_make_predictions

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