class_docstr: Returns the learning rate/s from the optimizer group/s. Use this to log the current LR when using decay.
  Default for key 'lr'
  
  State Requirements:
      - :attr:`torchbearer.state.OPTIMIZER`: The optimizer in state will be used to retrieve the learning rate.
code: def __init__(self):
      super(LR, self).__init__("lr")
  
file_path: torchbearer/torchbearer/metrics/lr.py
imports: import torchbearer
  from .metrics import AdvancedMetric
  from .decorators import default_for_key, to_dict
label_desc: Use image node_id 1 to create a new LR object from inherited base classes: AdvancedMetric with example: obj = LR()