Init the sinusoid position encoding table

def _position_encoding_init(max_length, dim):
    """Init the sinusoid position encoding table """
    position_enc = np.arange(max_length).reshape((-1, 1)) \
                   / (np.power(10000, (2. / dim) * np.arange(dim).reshape((1, -1))))
    # Apply the cosine to even columns and sin to odds.
    position_enc[:, 0::2] = np.sin(position_enc[:, 0::2])  # dim 2i
    position_enc[:, 1::2] = np.cos(position_enc[:, 1::2])  # dim 2i+1
    return position_enc