text: Title: resnet18_v1b_custom" is not among the following model list
  Body: Hi @bryanyzhu 
  
  I am trying to finetune resnet34_v1b_kinetics400 for 2 classes aby modifying the aataset part "resnet18_v1b_custom" to custom as described in the tutorial:
  
  https://cv.gluon.ai/build/examples_action_recognition/finetune_custom.html
  
  net = get_model(name='resnet18_v1b_custom', nclass=2)
  net.collect_params().reset_ctx(ctx)
  print(net)
  
  But got the below error:
  
  ---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  <ipython-input-9-ffe2680148eb> in <module>
        1 # net = get_model(name='i3d_resnet50_v1_custom', nclass=2)
  ----> 2 net = get_model(name='resnet18_v1b_custom', nclass=2)
        3 net.collect_params().reset_ctx(ctx)
        4 print(net)
  
  ~/anaconda3/lib/python3.8/site-packages/gluoncv/model_zoo/model_zoo.py in get_model(name, **kwargs)
      400         err_str = '"%s" is not among the following model list:\n\t' % (name)
      401         err_str += '%s' % ('\n\t'.join(sorted(_models.keys())))
  --> 402         raise ValueError(err_str)
      403     net = _models[name](**kwargs)
      404     return net
  
  ValueError: "resnet18_v1b_custom" is not among the following model list:
  	alexnet
  	alpha_pose_resnet101_v1b_coco
  	c3d_kinetics400
  	center_net_dla34_coco
  	center_net_dla34_dcnv2_coco
  
  Am I missing anything here ? How do we know which models are supported in the given list as I read it supports all the pretrained:
  
  https://cv.gluon.ai/model_zoo/action_recognition.html
  
  Thanks
  
  
  
  