class_docstr: Abstract base class used to build new callbacks.
  
  Subclass this class and override any of the relevant hooks
class_docstr_tok: ["Abstract","base","class","used","to","build","new","callbacks",".","Subclass","this","class","and","override","any","of","the","relevant","hooks"]
docstr: State key for checkpoints saved prior to version 1.5.0.
docstr_tok: ["State","key","for","checkpoints","saved","prior","to","version","1.5.0","."]
code: def _legacy_state_key(self) -> Type["Callback"]:
      """State key for checkpoints saved prior to version 1.5.0."""
      return type(self)
  
imports: from typing import Any, Dict, Type
  from torch import Tensor
  from torch.optim import Optimizer
  import lightning.pytorch
  from lightning.pytorch.utilities.types import STEP_OUTPUT
label_desc: Use image node_id 2 for calling the Callback obj's underlying member method code with example usage: obj._legacy_state_key() and returns: type