def METHOD_NAME(self, fields=None):
    """Retrieves specified fields as a dictionary of tensors.
    Args:
      fields: (optional) list of fields to return in the dictionary. If None
        (default), all fields are returned.
    Returns:
      tensor_dict: A dictionary of tensors specified by fields.
    Raises:
      ValueError: if specified field is not contained in boxlist.
    """
    tensor_dict = {}
    if fields is None:
        fields = self.get_all_fields()
    for field in fields:
        if not self.has_field(field):
            raise ValueError("boxlist must contain all specified fields")
        tensor_dict[field] = self.get_field(field)
    return tensor_dict