remove old torch dependency in requirements.txt

[tianyu-l]

Stack from ghstack (oldest at bottom):

• -> remove old torch dependency in requirements.txt #523

This has been discussed earlier -- let's remove official torch dependency in requirements.txt because

• it likely won't be useful, user will need a recent nightly anyway
• depending on nightly in requirements.txt is not ideal, e.g. (1) we cannot specify device type so packages for every device will be downloaded, which could be unnecessarily big; (2) the latest nightly may not work anyway because of new changes in torch