Create a batch for SG training objective with subwords.

def skipgram_fasttext_batch(centers, contexts, num_tokens, subword_lookup,
                            dtype, index_dtype):
    """Create a batch for SG training objective with subwords."""
    contexts = mx.nd.array(contexts[2], dtype=index_dtype)
    data, row, col = subword_lookup(centers)
    centers = mx.nd.array(centers, dtype=index_dtype)
    centers_csr = mx.nd.sparse.csr_matrix(
        (data, (row, col)), dtype=dtype,
        shape=(len(centers), num_tokens))  # yapf: disable
    return centers_csr, contexts, centers