Fix BerSumAbs Notebook

[daden-ms]

Description

abstractive_summarization_bertsumabs_cnndm notebook is outdated. Currently tested with torch==1.6.0 torchtext==0.9.1 torchvision==0.7.0 transformers==2.9.0

Related Issues

Checklist:

• [x] My code follows the code style of this project, as detailed in our contribution guidelines.
• [x] I have added tests.
• [x] I have updated the documentation accordingly.