Loads videos from files.

  Args:
    template: template string for listing the image files.
    video_length: length of the video.
    frame_shape: shape of each frame.

  Returns:
    dataset: the tf dataset frame by frame.
    dataset_len: number of the items which is the number of image files.

  Raises:
    ValueError: if no files found.

def load_videos(template, video_length, frame_shape):
  """Loads videos from files.

  Args:
    template: template string for listing the image files.
    video_length: length of the video.
    frame_shape: shape of each frame.

  Returns:
    dataset: the tf dataset frame by frame.
    dataset_len: number of the items which is the number of image files.

  Raises:
    ValueError: if no files found.
  """
  filenames = tf.gfile.Glob(template)
  if not filenames:
    raise ValueError("no files found.")
  filenames = sorted(filenames)
  dataset_len = len(filenames)
  filenames = tf.constant(filenames)
  dataset = tf.data.Dataset.from_tensor_slices(filenames)
  dataset = dataset.apply(tf.data.experimental.map_and_batch(
      lambda filename: load_image_map_function(filename, frame_shape),
      video_length, drop_remainder=True))
  return dataset, dataset_len