MNIST tutorial improvements. (#6232)
MNIST tutorial improvements. (#6232)

* Initial clean up of mnist tutorial

* More fixes + Image for MLP

* More content

* Image for CNN

* temporary: madjam's fork for images.

* temporary: madjam's fork for images.

* Image captions

* Formatting fix

* Format fix

* More content improvements

* Formatting fixes

* Content improvements

* code fix

* typo fix

* image borders

* fix img rendering

* fix img rendering

* Image box for mnist examples

* updates based on Bhavin's feedback

* training data -> train data

* Updates based on @zackchase feedback

Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet,Guneet-Dhillon/mxnet