Upgrade to pytorch 1.8 and python 3.7 and fix corresponding errors (#866)

* Upgrade to pytorch 1.8

* AdamW fix

* Remove trunc_normal

* Remove MixtureSameFamily

* Update doc requirements

* AdamW comment

* Remove unnecessary conversion

* Add torchtext

* Changed to python3.7; add torchtext

* Forwarding alf version number

* Fix rendering error in docker with python3.7

* The deadly triad for gym rendering: gym+pyglet+pillow

* More updates

* Add python3.7-dev in readme; minor carla docker update

* Minor fix