Build the vocab from corpus.

def collect_vocab(qp_pairs):
    '''
    Build the vocab from corpus.
    '''
    vocab = set()
    for qp_pair in qp_pairs:
        for word in qp_pair['question_tokens']:
            vocab.add(word['word'])
        for word in qp_pair['passage_tokens']:
            vocab.add(word['word'])
    return vocab