Implement shared hyperameters #69
Description
Currently, the hyperparameters of each primitive are specified individually.
However, there are some scenarios where hyperparameters from two different blocks should match each other.
An example of this are the keras.preprocessing.sequence.pad_sequences and keras.Sequential.LSTMTextClassifier primitives: The pad_sequences primitive has a maxlen hyperparameter that should be exactly the same value that is given to the LSTMTextClassifier as the input_length hyperparameter.