Create a batch for CBOW training objective with subwords.

def cbow_fasttext_batch(centers, contexts, num_tokens, subword_lookup, dtype,
                        index_dtype):
    """Create a batch for CBOW training objective with subwords."""
    _, contexts_row, contexts_col = contexts
    data, row, col = subword_lookup(contexts_row, contexts_col)
    centers = mx.nd.array(centers, dtype=index_dtype)
    contexts = mx.nd.sparse.csr_matrix(
        (data, (row, col)), dtype=dtype,
        shape=(len(centers), num_tokens))  # yapf: disable
    return centers, contexts