docstr: Model exponential moving average. 
docstr_tok: ["Model","exponential","moving","average","."]
code: def __init__(self, model, alpha=0.999):
      """Model exponential moving average."""
      self.step = 0
      self.model = model
      self.alpha = alpha
      self.shadow = self.get_model_state()  # init model
      self.backup = {}
      self.param_keys = [k for k, _ in self.model.named_parameters()]
      self.buffer_keys = [k for k, _ in self.model.named_buffers()]
  
file_path: PaddleSeg/contrib/DomainAdaptation/models/ema.py
label_desc: Use image node_id 1 to create a new EMA object from inherited base classes: object with example: obj = EMA(model, alpha)