code: def get_lr_at_epoch(cfg, cur_epoch):
      """
      Retrieve the learning rate of the current epoch with the option to perform
      warm up in the beginning of the training stage.
      Args:
          cfg (CfgNode): configs. Details can be found in
              slowfast/config/defaults.py
          cur_epoch (float): the number of epoch of the current training stage.
      """
      lr = get_lr_func(cfg.SOLVER.LR_POLICY)(cfg, cur_epoch)
      # Perform warm up.
      if cur_epoch < cfg.SOLVER.WARMUP_EPOCHS:
          lr_start = cfg.SOLVER.WARMUP_START_LR
          lr_end = get_lr_func(cfg.SOLVER.LR_POLICY)(
              cfg, cfg.SOLVER.WARMUP_EPOCHS
          )
          alpha = (lr_end - lr_start) / cfg.SOLVER.WARMUP_EPOCHS
          lr = cur_epoch * alpha + lr_start
      return lr
  
imports: import math
label_desc: Use image node_id 1 for calling a global function with example usage: get_lr_at_epoch(cfg, cur_epoch) and returns: lr