Various warnings in model from tutorial example.

Hi!

First big thanks on this project - seems very nice so far!

Just a quick one, I am seeing 3 warning when following the examples from the tutorials, they do not seem to affect the model in any obvious way, but I think it would be good to get them fixed up:

1. Layers in a Sequential model should only have a single input tensor

WARNING:tensorflow:Layers in a Sequential model should only have a single input tensor, but we receive a <class 'dict'> input: {'url': <tf.Tensor 'IteratorGetNext:2' shape=(None,) dtype=string>, 'text': <tf.Tensor 'IteratorGetNext:1' shape=(None,) dtype=string>}
Consider rewriting this model with the Functional API

2. dtype of the source tensor

WARNING:tensorflow:The dtype of the source tensor must be floating (e.g. tf.float32) when calling GradientTape.gradient, got tf.int32

3. Gradients do not exist for variables

WARNING:tensorflow:Gradients do not exist for variables ['counter:0'] when minimizing the loss.

If I get to the bottom of any of these issues I will be sure to report / PR.

Cheers!