text: Title: plot_network() doesn't support stock model generated by faster_rcnn_fpn_resnet50_v1b_coco()
  Body: It appears that function `gluoncv.utils.viz.plot_network` cannot recognise some stock models, e.g. the following code:
  
  ```
  import gluoncv
  import mxnet
  from mxnet.gluon import HybridBlock
  
  fpn: HybridBlock = gluoncv.model_zoo.faster_rcnn_fpn_resnet50_v1b_coco()
  fpn.hybridize()
  fpn.initialize(force_reinit=True)
  
  input = mxnet.ndarray.random.randn(1, 3, 400, 400)
  
  output = fpn.forward(input)
  
  for i in output:
      print(i.shape)
  
  shape = input.shape
  gluoncv.utils.viz.plot_network(fpn, shape=shape)
  ```
  
  will results in the following error:
  
  ```
  KeyError                                  Traceback (most recent call last)
  
  <ipython-input-2-07afc689e980> in testRun(self)
       17 
       18         shape = input.shape
  ---> 19         gluoncv.utils.viz.plot_network(fpn, shape=shape)
       20 
       21 
  
  /home/shared/venv/cv_gluon/lib/python3.7/site-packages/gluoncv/utils/viz/network.py in plot_network(block, shape, save_prefix)
       30         sym = mx.sym.Group(sym)
       31     a = mx.viz.plot_network(sym, shape={'data':shape},
  ---> 32                             node_attrs={'shape':'rect', 'fixedsize':'false'})
       33     a.view(tempfile.mktemp('.gv'))
       34     if isinstance(save_prefix, str):
  
  /home/shared/venv/cv_gluon/lib/python3.7/site-packages/mxnet/visualization.py in plot_network(symbol, title, save_format, shape, dtype, node_attrs, hide_weights)
      402                                 if "num_outputs" in params:
      403                                     key += str(int(params["num_outputs"]) - 1)
  --> 404                             shape = shape_dict[key][1:]
      405                             label = "x".join([str(x) for x in shape])
      406                             attr["label"] = label
  
  KeyError: 'fasterrcnn0_rpn0_box_nms0_output'
  ```
  