train_mnist.py in example/image-classification does not work with inception-bn and resnet

[roywei]

Description

train_mnist.py in example/image-classification does not work with inception-bn and resnet
As in title, the following commands will fail:

python train_mnist.py --network inception-bn --gpus 0,1,2,3

python train_mnist.py --network resnet --num-layers 110  --gpus 0,1,2,3

Error Message:

Traceback (most recent call last):
  File "train_mnist.py", line 93, in <module>
    sym = net.get_symbol(**vars(args))
TypeError: get_symbol() takes at least 3 arguments (3 given)

Root cause

The get_symbol() in those models requires additional image_shape argument, and train_mnist.py does not provide it

[roywei]

@mxnet-label-bot add [Python, Example]