GPflow makes use of a multipledispatch Dispatcher that internally uses a generator.  However, according to TensorFlow [Capabilities and Limitations](https://github.com/tensorflow/tensorflow/blob/560e2575ecad30bedff5b192f33f6d06b19ccaeb/tensorflow/python/autograph/LIMITATIONS.md) generators are not supported by AutoGraph and probably will never be. Thus, compiling code that passed though the dispatcher led to the following warnings:
```bash
WARNING:tensorflow:Entity <bound method Dispatcher.dispatch_iter of <dispatched sample_conditional>> appears to be a generator function. It will not be converted by AutoGraph.
WARNING: Entity <bound method Dispatcher.dispatch_iter of <dispatched sample_conditional>> appears to be a generator function. It will not be converted by AutoGraph.
WARNING:tensorflow:Entity <bound method Dispatcher.dispatch_iter of <dispatched conditional>> appears to be a generator function. It will not be converted by AutoGraph.
WARNING: Entity <bound method Dispatcher.dispatch_iter of <dispatched conditional>> appears to be a generator function. It will not be converted by AutoGraph.
```

This PR still uses the same dispatcher, but overwrites the problematic method that uses python generators and replaces it by a simple list.


### NOTE
As of this PR we do not need to write `autograph=False` in `tf.function` anymore, and all the code inside dispatching gets compiled the same way as everything else :) !