NotImplementedError: Layers with arguments in `__init__` must override `get_config`

It's not a bug, it's a feature.

This error lets you know that TF can't save your model, because it won't be able to load it.
Specifically, it won't be able to reinstantiate your custom Layer classes: encoder and decoder.

To solve this, just override their get_config method according to the new arguments you've added.

A layer config is a Python dictionary (serializable) containing the configuration of a layer. The same layer can be reinstantiated later (without its trained weights) from this configuration.

For example, if your encoder class looks something like this:

class encoder(tf.keras.layers.Layer):

    def __init__(
        self,
        vocab_size, num_layers, units, d_model, num_heads, dropout,
        **kwargs,
    ):
        super().__init__(**kwargs)
        self.vocab_size = vocab_size
        self.num_layers = num_layers
        self.units = units
        self.d_model = d_model
        self.num_heads = num_heads
        self.dropout = dropout

    # Other methods etc.

then you only need to override this method:

    def get_config(self):

        config = super().get_config().copy()
        config.update({
            'vocab_size': self.vocab_size,
            'num_layers': self.num_layers,
            'units': self.units,
            'd_model': self.d_model,
            'num_heads': self.num_heads,
            'dropout': self.dropout,
        })
        return config

When TF sees this (for both classes), you will be able to save the model.

Because now when the model is loaded, TF will be able to reinstantiate the same layer from config.

Layer.from_config's source code may give a better sense of how it works:

@classmethod
def from_config(cls, config):
  return cls(**config)

This problem is caused by mixing imports between the keras and tf.keras libraries, which is not supported.

Use tf.keras.models or usr keras.models everywhere

You should never mix imports between these libraries, as it will not work and produces all kinds of strange error messages. These errors change with versions of keras and tensorflow.