'Pad the document with the place holder according to the window_size'

@classmethod
def _build_new_documents(cls, documents, window_size, pad_left=False, pad_right=False, pad_symbol=None):

    padding = ((pad_symbol,) * (window_size - 1))
    if pad_right:
        return _itertools.chain.from_iterable((_itertools.chain(doc, padding) for doc in documents))
    if pad_left:
        return _itertools.chain.from_iterable((_itertools.chain(padding, doc) for doc in documents))
