Refactor ECG notebooks for Keras torch backend

[madhavajay]

Summary

• switch ECG notebooks to Keras with the PyTorch backend and drop TensorFlow from test installs
• add pyfory policy hook compatibility for validate_module/validate_method
• fix DO notebook request flow after publishing the ecg twin

Testing

• not run here (user reported manual + CLI pass: ./test_notebooks_syftbox.sh notebooks/06-ecg.json)

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