Update fit.py (apache#10685)

* Update fit.py

args.network is None during finetuing using fine-tune.py file which is provided. Hence, line 241 throws an error while trying to see if 'vgg' is in args.network. Patching it by checking if args.network is None before seeing if 'vgg' is present in network name.

* Update fit.py

example/image-classification/common/fit.py

@@ -238,7 +238,7 @@ def fit(args, network, data_loader, **kwargs):
             # AlexNet will not converge using Xavier
             initializer = mx.init.Normal()
             # VGG will not trend to converge using Xavier-Gaussian
-        elif 'vgg' in args.network:
+        elif args.network and 'vgg' in args.network:
             initializer = mx.init.Xavier()
         else:
             initializer = mx.init.Xavier(