def add_field(self, field, field_data):
    """Add data to a specified field.

    Args:
      field: a string parameter used to speficy a related field to be accessed.
      field_data: a numpy array of [N, ...] representing the data associated
          with the field.
    Raises:
      ValueError: if the field is already exist or the dimension of the field
          data does not matches the number of boxes.
    """
    if self.has_field(field):
      raise ValueError('Field ' + field + 'already exists')
    if len(field_data.shape) < 1 or field_data.shape[0] != self.num_boxes():
      raise ValueError('Invalid dimensions for field data')
    self.data[field] = field_data
Add data to a specified field.

    Args:
      field: a string parameter used to speficy a related field to be accessed.
      field_data: a numpy array of [N, ...] representing the data associated
          with the field.
    Raises:
      ValueError: if the field is already exist or the dimension of the field
          data does not matches the number of boxes.
    
inference_api/src/main/object_detection/utils/np_box_list.py