Gets default bottom transformation; if none available, return value.

def get_bottom(modality_type, value=None):
  """Gets default bottom transformation; if none available, return value."""
  if modality_type == ModalityType.AUDIO:
    return audio_bottom
  elif modality_type == ModalityType.AUDIO_SPECTRAL:
    return audio_spectral_bottom
  elif modality_type in (ModalityType.CLASS_LABEL,
                         ModalityType.MULTI_LABEL,
                         ModalityType.ONE_HOT_CLASS_LABEL,
                         ModalityType.SIGMOID_CLASS_LABEL,
                         ModalityType.SIGMOID_MAX_POOLING_CLASS_LABEL,
                         ModalityType.SOFTMAX_AVERAGE_POOLING_CLASS_LABEL,
                         ModalityType.SOFTMAX_LAST_TIMESTEP_CLASS_LABEL,
                         ModalityType.SOFTMAX_MAX_POOLING_CLASS_LABEL):
    return class_label_bottom
  elif modality_type in (ModalityType.CTC_SYMBOL,
                         ModalityType.SYMBOL,
                         ModalityType.SYMBOL_WEIGHTS_ALL):
    return symbol_bottom
  elif modality_type in (ModalityType.GENERIC_L2_LOSS,
                         ModalityType.IDENTITY,
                         ModalityType.IDENTITY_SYMBOL,
                         ModalityType.IMAGE_CHANNEL_EMBEDDINGS_BOTTOM):
    return identity_bottom
  elif modality_type == ModalityType.IMAGE:
    return image_bottom
  elif modality_type in (ModalityType.IMAGE_CHANNEL_BOTTOM_IDENTITY,
                         ModalityType.IMAGE_CHANNEL_COMPRESS):
    return image_channel_compress_bottom
  elif modality_type in (ModalityType.REAL,
                         ModalityType.REAL_L2_LOSS,
                         ModalityType.REAL_LOG_POISSON_LOSS):
    return real_bottom
  elif modality_type == ModalityType.SPEECH_RECOGNITION:
    return speech_recognition_bottom
  elif modality_type == ModalityType.SYMBOL_ONE_HOT:
    return symbol_one_hot_bottom
  elif modality_type in (ModalityType.VIDEO,
                         ModalityType.VIDEO_L1,
                         ModalityType.VIDEO_L2):
    return video_bottom
  elif modality_type == ModalityType.VIDEO_BITWISE:
    return video_bitwise_bottom
  elif modality_type == ModalityType.VIDEO_IDENTITY:
    return video_identity_bottom
  elif modality_type in (ModalityType.VIDEO_L1_RAW,
                         ModalityType.VIDEO_L2_RAW):
    return video_raw_bottom
  elif modality_type == ModalityType.VIDEO_PIXEL_NOISE:
    return video_pixel_noise_bottom
  return value