text: Title: The problem of SiamMask training
  Body: Hi, when I try to train SiamMask, there is an output error: 
      File "train_pysot.py", line 106, in build_opt_lr
      'lr': cfg.TRAIN.LR.BASE_LR}]
    File "/home/wqq/anaconda3/envs/pysot/lib/python3.7/site-packages/yacs/config.py", line 141, in __getattr__
      raise AttributeError(name)
  AttributeError: BASE_LR
  It seems that there is something wrong in the config.yaml file in experiments/siammask_r50_l3. Any suggestions? Thanks. 