Showcase "transfer learning" with imagenet example

[andsteing]

The existing examples/imagenet/notebook.ipynb could be extended to showcase how to do transfer learning, i.e. use a pre-trained imagenet model, remove some layers, and then train with some other layers frozen.

#1565

[marcvanzee]

@cgarciae who is working on a transfer learning example

[marcvanzee]

@andsteing given that Cristian created a Transfer learning example, I am closing this issue but feel free to re-open if you are still planning on adding it to the Colab (just trying to trim down our issues a bit).