Makes a module spec to simply perform token to embedding lookups.

  Input of this module is a 1-D list of string tokens. For T tokens input and
  an M dimensional embedding table, the lookup result is a [T, M] shaped Tensor.

  Args:
    vocabulary_file: Text file where each line is a key in the vocabulary.
    vocab_size: The number of tokens contained in the vocabulary.
    embeddings_dim: The embedding dimension.
    num_oov_buckets: The number of out-of-vocabulary buckets.
    preprocess_text: Whether to preprocess the input tensor by removing
      punctuation and splitting on spaces.

  Returns:
    A module spec object used for constructing a TF-Hub module.

def make_module_spec(vocabulary_file, vocab_size, embeddings_dim,
                     num_oov_buckets, preprocess_text):
  """Makes a module spec to simply perform token to embedding lookups.

  Input of this module is a 1-D list of string tokens. For T tokens input and
  an M dimensional embedding table, the lookup result is a [T, M] shaped Tensor.

  Args:
    vocabulary_file: Text file where each line is a key in the vocabulary.
    vocab_size: The number of tokens contained in the vocabulary.
    embeddings_dim: The embedding dimension.
    num_oov_buckets: The number of out-of-vocabulary buckets.
    preprocess_text: Whether to preprocess the input tensor by removing
      punctuation and splitting on spaces.

  Returns:
    A module spec object used for constructing a TF-Hub module.
  """

  def module_fn():
    """Spec function for a token embedding module."""
    tokens = tf.placeholder(shape=[None], dtype=tf.string, name="tokens")

    embeddings_var = tf.get_variable(
        initializer=tf.zeros([vocab_size + num_oov_buckets, embeddings_dim]),
        name=EMBEDDINGS_VAR_NAME,
        dtype=tf.float32)

    lookup_table = tf.contrib.lookup.index_table_from_file(
        vocabulary_file=vocabulary_file,
        num_oov_buckets=num_oov_buckets,
    )
    ids = lookup_table.lookup(tokens)
    combined_embedding = tf.nn.embedding_lookup(params=embeddings_var, ids=ids)
    hub.add_signature("default", {"tokens": tokens},
                      {"default": combined_embedding})

  def module_fn_with_preprocessing():
    """Spec function for a full-text embedding module with preprocessing."""
    sentences = tf.placeholder(shape=[None], dtype=tf.string, name="sentences")
    # Perform a minimalistic text preprocessing by removing punctuation and
    # splitting on spaces.
    normalized_sentences = tf.regex_replace(
        input=sentences, pattern=r"\pP", rewrite="")
    tokens = tf.string_split(normalized_sentences, " ")

    embeddings_var = tf.get_variable(
        initializer=tf.zeros([vocab_size + num_oov_buckets, embeddings_dim]),
        name=EMBEDDINGS_VAR_NAME,
        dtype=tf.float32)
    lookup_table = tf.contrib.lookup.index_table_from_file(
        vocabulary_file=vocabulary_file,
        num_oov_buckets=num_oov_buckets,
    )
    sparse_ids = tf.SparseTensor(
        indices=tokens.indices,
        values=lookup_table.lookup(tokens.values),
        dense_shape=tokens.dense_shape)

    # In case some of the input sentences are empty before or after
    # normalization, we will end up with empty rows. We do however want to
    # return embedding for every row, so we have to fill in the empty rows with
    # a default.
    sparse_ids, _ = tf.sparse_fill_empty_rows(
        sparse_ids, lookup_table.lookup(tf.constant("")))
    # In case all of the input sentences are empty before or after
    # normalization, we will end up with a SparseTensor with shape [?, 0]. After
    # filling in the empty rows we must ensure the shape is set properly to
    # [?, 1]. At this point, there are no empty rows, so the new shape will be
    # [sparse_ids.dense_shape[0], max(1, sparse_ids.dense_shape[1])].
    sparse_ids = tf.sparse_reset_shape(sparse_ids)

    combined_embedding = tf.nn.embedding_lookup_sparse(
        params=embeddings_var,
        sp_ids=sparse_ids,
        sp_weights=None,
        combiner="sqrtn")

    hub.add_signature("default", {"sentences": sentences},
                      {"default": combined_embedding})

  if preprocess_text:
    return hub.create_module_spec(module_fn_with_preprocessing)
  else:
    return hub.create_module_spec(module_fn)