Missing assert statement: Nbeats 'n_hidden' argument

[alejandroxag]

When instantiating a Nbeats model, the init() argument 'n_hidden' must be a list of lists, one per 'stack_type,' so each corresponding set of NBeatsBlock is initialized passing a list as the 'theta_n_hidden' argument.

An assert statement in Nbeats.init() would help the user identify if this argument is properly stated, particularly when running grid routines (hyper optimization or ensembling), as the 'n_hidden' item in the grid needs to be defined as a list of lists of lists.

[kdgutier]

I think it would be convenient to change the n_hidden name to n_hidden_list to make it more informative.