[Dynamic RPC] Allow for optional world_size argument in init_rpc (#73372)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73372
This PR which allows for optional `world_size` argument in init_rpc. This makes changes in rendezvous to allow for `NoneType` for world_size and creates a new code path when initializing TensorPipe agent for init_rpc. The TensorPipe agent is protected by a critical section enforced using the store, so that only one node can create a TPAgent at a time.
This PR does not yet enable RPC commands between ranks.
Previously:
```python
os.environ['MASTER_ADDR'] = 'localhost'
os.environ['MASTER_PORT'] = '29500'
init_rpc("worker0", world_size=1, rank=0)
```
Now (only rank is needed):
```python
os.environ['MASTER_ADDR'] = 'localhost'
os.environ['MASTER_PORT'] = '29500'
init_rpc("worker0", rank=0)
```
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D34621651
Pulled By: H-Huang
fbshipit-source-id: 09dbb511d5a00c219a6ce0a35501ff2e388998b0
(cherry picked from commit 834aedc3256167399c323169ef2f0c9b3cf98dff)
