Fix BERT tutorial img rendering

Summary:
Img rendering works fine on github (https://github.com/pytorch/opacus/blob/master/tutorials/building_text_classifier.ipynb), but not on the website (https://opacus.ai/tutorials/building_text_classifier)

Here I'm switching from rending as markdown cell to using `IPython.display.Image` (It works for guys at Captum and our website is similar).

Reviewed By: ashkan-software

Differential Revision: D23455492

fbshipit-source-id: 027dc0b4f55399ff2c66eddd595c8198b9b7fd94

Author: Igor Shilov