Error when loading models with no params

[samskalicky]

When loading small unit test models (such as models that contain only an identity operator) there may not be any params in the params file. When this happens the "save_dict" is an empty list rather than a dict. In load_checkpoint an error is thrown when we call "*.items()" on a list when its expecting a dict.

https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/model.py#L451-L454

We need to add a check before the for loop to avoid this problem

Example code

import mxnet as mx

a = mx.sym.var('data')
s = mx.sym.identity(a)

mod = mx.mod.Module(s)
mod.bind(for_training=False, data_shapes=[('data',(1,))])
mod.init_params()
mod.save_checkpoint('identity',0)

s,a,x = mx.model.load_checkpoint('identity',0)

Error Message

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/mxnet/model.py", line 452, in load_checkpoint
    for k, v in save_dict.items():
AttributeError: 'list' object has no attribute 'items'

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