code: def _format_metric(prefix, metric, id=-1):
      """Format the metric as test/mean_squared_error, or val/mean_squared_error."""
      return "{}/{}".format(prefix, metric.__name__)
  
imports: from bigdl.nano.pytorch.lightning import LightningModule
  from bigdl.nano.utils.common import invalidInputError
  from typing import List
  import pytorch_lightning
  from torchmetrics.metric import Metric
  from torch.optim.lr_scheduler import _LRScheduler
  import bigdl.nano.automl.hpo
  import warnings
  import torch
  import inspect
  from torch import nn, Tensor, fx
  from torch.utils.data import TensorDataset, DataLoader
  from bigdl.nano.automl.hpo.space import Space
label_desc: Use image node_id 2 for calling a global function with example usage: _format_metric(prefix, metric, id) and returns: str