class_docstr: Test individual Metric classes.
class_docstr_tok: ["Test","individual","Metric","classes","."]
code: def test_fixedmetric(self):
      assert (FixedMetric(3) + FixedMetric(3)).value() == 3
      with self.assertRaises(ValueError):
          _ = FixedMetric(3) + FixedMetric(4)
  
imports: import unittest
  from typing import Dict
  import torch
  import random
  from parlai.core.metrics import AverageMetric, SumMetric, FixedMetric, Metrics, GlobalAverageMetric, MacroAverageMetric, TimerMetric, aggregate_unnamed_reports, aggregate_named_reports, InterDistinctMetric, IntraDistinctMetric, FairseqBleuMetric
  from parlai.core.torch_classifier_agent import ConfusionMatrixMetric, WeightedF1Metric, AUCMetrics
  from parlai.core.torch_generator_agent import PPLMetric
  import parlai.utils.testing
label_desc: Use image node_id 5 for calling the TestMetric obj's underlying member method code with example usage: obj.test_fixedmetric() without return types