Mnist with Tensorflow Dataset Api

[YanYas]

Hi @migueldeicaza, I've been struggling to setup datasets for batching with the Tensorflow's internal dataset system.

I would say the problems lie with the Iterator where it seems to get confused about the compnents of an array when receiving a TensorDataset. It is working fine with dataset generators, such as RangeDataset. Could the Mnist example be updated to show how the file-based datasets correctly implement the dataset api for batching and shuffling?

And thanks of course for your work so far.

[miroslav22]

#390

Likely to be fixed by 390