Modify AG_NEWS import to work under new and old torchtext #142 (#143)
* Modify AG_NEWS import to work under new and old torchtext
torchtext >=0.8.0 moved AG_NEWS from experimental to mainline
* Update torchbenchmark/models/fastNLP/__init__.py
Co-authored-by: Nikita Shulga <nshulga@fb.com>