class_docstr: Number of unique utterances.
class_docstr_tok: ["Number","of","unique","utterances","."]
code: def value(self) -> int:
      if not self._counter:
          return 0
      return len(self._counter)
  
imports: from __future__ import annotations
  from typing import Tuple
  from collections import Counter
  from parlai.core.torch_agent import TorchAgent
  from parlai.core.metrics import Metric, SumMetric
  from parlai.core.message import Message
label_desc: Use image node_id 1 for calling the UniqueMetric obj's underlying member method code with example usage: obj.value() and returns: len, int