[BE]Delay the call to set_pytorch_distributed_envs_from_justknobs (#120625)
When initializing the default process group, `init_process_group` will show the explicit message indicating the default process group is being initialized twice.
However, with `set_pytorch_distributed_envs_from_justknobs` being the very first line in `init_process_group`, the error message becomes implicit and hard to understand the root cause when testing with the FB code base.
Differential Revision: [D54206202](https://our.internmc.facebook.com/intern/diff/D54206202/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120625
Approved by: https://github.com/wconstab, https://github.com/yifuwang
