Masking Support

Hi @thushv89,

Thank you for your code. And the blog post describing it [here](https://towardsdatascience.com/light-on-math-ml-attention-with-keras-dc8dbc1fad39).
I wonder if your implementation supports masking?

In Keras, if mask_zero=True is set in the embeddings layer then proper masking should be supported by the consecutive layers.
Any thoughts?

Best Regards,
Omnia