class_docstr: StateKey class that is a unique state key based on the input string key. State keys are also metrics which
  retrieve themselves from state.
  
  Args:
      key (str): Base key
class_docstr_tok: ["StateKey","class","that","is","a","unique","state","key","based","on","the","input","string","key",".","State","keys","are","also","metrics","which","retrieve","themselves","from","state",".","Args",":","key","(","str",")",":","Base","key"]
code: def __call__(self, state):
      return state[self]
  
imports: from torchbearer import Metric
  import warnings
label_desc: Use image node_id 4 for calling the StateKey obj's underlying member method code with example usage: obj.__call__(state) and returns: state