doc updates for TorchScript (#16866)
Summary:
Some batched updates:
1. bool is a type now
2. Early returns are allowed now
3. The beginning of an FAQ section with some guidance on the best way to do GPU training + CPU inference
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16866
Differential Revision: D13996729
Pulled By: suo
fbshipit-source-id: 3b884fd3a4c9632c9697d8f1a5a0e768fc918916
