Latent prediction and loss.

def ae_latent_softmax(latents_pred, latents_discrete, hparams):
  """Latent prediction and loss."""
  vocab_size = 2 ** hparams.z_size
  if hparams.num_decode_blocks < 2:
    latents_logits = tf.layers.dense(latents_pred, vocab_size,
                                     name="extra_logits")
    if hparams.logit_normalization:
      latents_logits *= tf.rsqrt(1e-8 +
                                 tf.reduce_mean(tf.square(latents_logits)))

    loss = None
    if latents_discrete is not None:
      if hparams.soft_em:
        # latents_discrete is actually one-hot of multinomial samples
        assert hparams.num_decode_blocks == 1
        loss = tf.nn.softmax_cross_entropy_with_logits_v2(
            labels=latents_discrete, logits=latents_logits)
      else:
        loss = tf.nn.sparse_softmax_cross_entropy_with_logits(
            labels=latents_discrete, logits=latents_logits)
    sample = multinomial_sample(
        latents_logits, vocab_size, hparams.sampling_temp)
    return sample, loss

  # Multi-block case.
  vocab_bits = int(math.log(vocab_size, 2))
  assert vocab_size == 2**vocab_bits
  assert vocab_bits % hparams.num_decode_blocks == 0
  block_vocab_size = 2**(vocab_bits // hparams.num_decode_blocks)
  latents_logits = [
      tf.layers.dense(
          latents_pred, block_vocab_size, name="extra_logits_%d" % i)
      for i in range(hparams.num_decode_blocks)
  ]
  loss = None
  if latents_discrete is not None:
    losses = []
    for i in range(hparams.num_decode_blocks):
      d = tf.floormod(tf.floordiv(latents_discrete,
                                  block_vocab_size**i), block_vocab_size)
      losses.append(tf.nn.sparse_softmax_cross_entropy_with_logits(
          labels=d, logits=latents_logits[i]))
    loss = sum(losses)
  samples = [multinomial_sample(l, block_vocab_size, hparams.sampling_temp)
             for l in latents_logits]
  sample = sum([s * block_vocab_size**i for i, s in enumerate(samples)])
  return sample, loss