add support for mock data input to imagenet example

[suiyuan2009]

in order to test the performance conveniently, especially distributed training, it would be great to add an option to allow mock data input, which means directly random some data in program as input.

[subramen]

@suiyuan2009 it's been a while since you posted this, but in the chance that you're still interested in this I can help you with raising a PR for this