@abstractmethod<EOL><INDENT>def fit(self, item_users):<DEDENT>
pass<EOL>
Trains the model on a sparse matrix of item/user/weight

Parameters
----------
item_user : csr_matrix
    A matrix of shape (number_of_items, number_of_users). The nonzero
    entries in this matrix are the items that are liked by each user.
    The values are how confidant you are that the item is liked by the user.