modify runtime/image_classification/models/resnet50/gpus=2/__init__.py

[SHEELE41]

Hello, I found some bug in resnet50 gpus 2 experiment code.
A keyerror was raised when running the experiment.

Traceback (most recent call last):
  File "main_with_runtime.py", line 585, in <module>
    main()
  File "main_with_runtime.py", line 146, in main
    input_tensor = torch.zeros(tuple(training_tensor_shapes[input]),
KeyError: 'input'

So I modified runtime/image_classification/models/resnet50/gpus=2/init.py file and saw it works.