text: Title: dump-model-params.py fail for Fasterrcnn horovod checkpt
  Body: If you're asking about an unexpected problem you met, use this template.
  __PLEASE DO NOT DELETE THIS TEMPLATE, FILL IT__:
  
  ### 1. What you did:
  
  (1) **If you're using examples, what's the command you run:**
  dump-model-params.py --meta mymeta myinputchpt outputnpz.npz
  
  (2) **If you're using examples, have you made any changes to the examples? Paste `git diff` here:**
  
  (3) **If not using examples, tell us what you did:**
  
  Note that we may not be able to investigate it if there is no reproducible code.
  It's always better to paste what you did instead of describing them.
  
  ### 2. What you observed:
  
  (1) **Include the ENTIRE logs here:**
  
  Traceback (most recent call last):
   File "dump-model-params.py", line 25, in <module>
     tf.train.import_meta_graph(args.meta, clear_devices=True)
   File "/home/ubuntu/anaconda3/envs/tensorflow_p36/lib/python3.6/site-packages/tensorflow/python/training/saver.py", line 1674, in import_meta_graph
     meta_graph_or_file, clear_devices, import_scope, **kwargs)[0]
   File "/home/ubuntu/anaconda3/envs/tensorflow_p36/lib/python3.6/site-packages/tensorflow/python/training/saver.py", line 1696, in _import_meta_graph_with_return_elements
     **kwargs))
   File "/home/ubuntu/anaconda3/envs/tensorflow_p36/lib/python3.6/site-packages/tensorflow/python/framework/meta_graph.py", line 806, in import_scoped_meta_graph_with_return_elements
     return_elements=return_elements)
   File "/home/ubuntu/anaconda3/envs/tensorflow_p36/lib/python3.6/site-packages/tensorflow/python/util/deprecation.py", line 488, in new_func
     return func(*args, **kwargs)
   File "/home/ubuntu/anaconda3/envs/tensorflow_p36/lib/python3.6/site-packages/tensorflow/python/framework/importer.py", line 391, in import_graph_def
     _RemoveDefaultAttrs(op_dict, producer_op_list, graph_def)
   File "/home/ubuntu/anaconda3/envs/tensorflow_p36/lib/python3.6/site-packages/tensorflow/python/framework/importer.py", line 158, in _RemoveDefaultAttrs
     op_def = op_dict[node.op]
  KeyError: 'HorovodAllreduce'
  
  It's always better to paste what you observed instead of describing them.
  
  It's always better to paste **as much as possible**, although sometimes a partial log is OK.
  
  Tensorpack typically saves stdout to its training log.
  If stderr is relevant, you can run a command with `CMD 2>&1 | tee logs.txt`
  to save both stdout and stderr to one file.
  
  (2) **Other observations, if any:**
  For example, CPU/GPU utilization, output images, tensorboard curves, if relevant to your issue.
  
  ### 3. What you expected, if not obvious.
  
  If you expect higher speed, please first read http://tensorpack.readthedocs.io/en/latest/tutorial/performance-tuning.html
  
  If you expect higher accuracy, only in one of the two conditions can we help with it:
  (1) You're unable to match the accuracy documented in tensorpack examples.
  (2) It appears to be a tensorpack bug.
  
  Otherwise, how to get high accuracy is a machine learning question and is
  not our responsibility to figure out.
  
  ### 4. Your environment:
    + Python version:
    + TF version: `python -c 'import tensorflow as tf; print(tf.GIT_VERSION, tf.VERSION)'`.
    + Tensorpack version: `python -c 'import tensorpack; print(tensorpack.__version__);'`.
        You can install Tensorpack master by `pip install -U git+https://github.com/ppwwyyxx/tensorpack.git`
        and see if your issue is already solved.
    + If you're not using tensorpack under a normal command line shell (e.g.,
      using an IDE or jupyter notebook), please retry under a normal command line shell. 
    + Hardware information, e.g. number of GPUs used.
  
  Feel free to add extra information related to your issue, but
  please try to provide the above information __accurately__ to save effort in the investigation.
  