Add new keras.Sequential primitives

[csala]

The Keras documentation has a collection of example models built using the keras.Sequential class: https://keras.io/getting-started/sequential-model-guide/#examples

We should review them and see if we can have a primitives for each one of them.

If there is any change required in the adapter or new datasets are needed for testing, the required changes can be covered in this issue.

Also add demo pipelines for each one of the already existing primitives.

Consider this issue a "meta-issue", which can be divided in several smaller issues, one per primitive.

[csala]

More primitives can be found here