class_docstr: The exception type raised if an `on_backward` callback
  attempts to call `backward` when `backward_called` is `True`.
code: def __init__(self, message="") -> None:
      super().__init__(
          "Backpropagation has already been performed"
          "and the computation graph has been erased, so"
          "calling `loss.backward` is not permitted. " + message
      )
  
file_path: allennlp/allennlp/training/callbacks/backward.py
imports: from typing import Dict, TYPE_CHECKING
  import torch
  from allennlp.training.callbacks.callback import TrainerCallback
label_desc: Use image node_id 1 to create a new OnBackwardException object from inherited base classes: Exception with example: obj = OnBackwardException(message)