Encode Text2Text samples from the generator with the vocab.

def text2text_generate_encoded(sample_generator,
                               vocab,
                               targets_vocab=None,
                               has_inputs=True,
                               inputs_prefix="",
                               targets_prefix=""):
  """Encode Text2Text samples from the generator with the vocab."""
  targets_vocab = targets_vocab or vocab
  for sample in sample_generator:
    if has_inputs:
      sample["inputs"] = vocab.encode(inputs_prefix + sample["inputs"])
      sample["inputs"].append(text_encoder.EOS_ID)
    sample["targets"] = targets_vocab.encode(targets_prefix + sample["targets"])
    sample["targets"].append(text_encoder.EOS_ID)
    yield sample