class_docstr: Represents an output of a model that can be served.
  
  These typically correspond to model heads.
class_docstr_tok: ["Represents","an","output","of","a","model","that","can","be","served",".","These","typically","correspond","to","model","heads","."]
code: def _check_output_key(self, key, error_label):
      # For multi-head models, the key can be a tuple.
      if isinstance(key, tuple):
          key = self._SEPARATOR_CHAR.join(key)
  
      if not isinstance(key, str):
          raise ValueError(
              "{} output key must be a string; got {}.".format(
                  error_label, key
              )
          )
      return key
  
imports: import abc
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor
  from tensorflow.python.framework import tensor_util
  from tensorflow.python.keras.saving.utils_v1 import signature_def_utils
  from tensorflow.python.saved_model import signature_def_utils
label_desc: Use image node_id 2 for calling the ExportOutput obj's underlying member method code with example usage: obj._check_output_key(key, error_label) and returns: key