Demonstrate best practices in the notebooks: @tf.function

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Documentation/tutorial notebooks

Are there any mistakes in the docs?
Some notebooks (e.g. https://gpflow.github.io/GPflow/develop/notebooks/advanced/natural_gradients.html, there may be more, I did not check extensively) do not use @tf.function when using a non-Scipy optimizer (our Scipy class automatically wraps the optimisation in tf.function by default). This is significantly slower in practice, and for any real data sets you would want to ensure the tf.function-precompilation. The notebooks should demonstrate this best practice, so users do not wonder why GPflow is so slow...