Document debugability features in torch.distributed (#59604)
Summary:
Adds comprehensive documentation around debugability features added to `torch.distributed` recently, including the `monitored_barrier` and TORCH_DISTRIBUTED_DEBUG env variable.
![dist_one](https://user-images.githubusercontent.com/8039770/121102672-0f052180-c7b3-11eb-974c-81dbbe102cb6.png)
![dist_two](https://user-images.githubusercontent.com/8039770/121102734-39ef7580-c7b3-11eb-94f7-c75469351440.png)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59604
Reviewed By: jbschlosser, SciPioneer
Differential Revision: D28972444
Pulled By: rohan-varma
fbshipit-source-id: da5e8ee84f0d6f252c703c4d70ff2a0d5817cc4e