Fix undefined error in image segmentation

ctx is used undefined. Setting the default ctx to cpu and
editing the comment to let the user know that it can be
changed to GPU as required.

example/fcn-xs/image_segmentaion.py

@@ -93,8 +93,8 @@ def main():
     model_prefix = "FCN8s_VGG16"
     epoch = 19
 
-    # By default, MXNet will run on the CPU. Uncomment the line below to execute on the GPU
-    # ctx = mx.gpu()
+    # By default, MXNet will run on the CPU. Change to ctx = mx.gpu() to run on GPU.
+    ctx = mx.cpu()
 
     fcnxs, fcnxs_args, fcnxs_auxs = mx.model.load_checkpoint(model_prefix, epoch)
     fcnxs_args["data"] = mx.nd.array(get_data(args.input), ctx)