Make init_method url appending step re-usable by both init_process_group and init_model_parallel(init_rpc) (#28226)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28226
# Goal
Rendezvous step should be the first step not only for `init_process_group` but also for `init_model_parallel`.
The road block is that there is special step in `init_process_group` where arguments `rank`, `world_size` passed to `init_process_group(..)` are appended to `init_method` url string.
We need to make this argument appending step common and re-usable for both `init_process_group` and `init_model_parallel`.
# Solution
- Put argument appending inside of `rendezvous` function.
- Remove manual `init_method` url construction. Delegate the responsibility to the `rendezvous` function.
- Use the `rendezvous` function for any `RpcAgent`.
Test Plan:
```
buck test mode/dev-nosan caffe2/test:c10d
```
```
buck test mode/dev-nosan caffe2/test:rpc_fork -- test_invalid_names
buck-out/gen/caffe2/test/rpc_fork\#binary.par -r test_worker_id
```
```
buck test mode/dev-nosan caffe2/torch/fb/distributed/pytorch/tests:test_rpc -- test_sync_rpc
```
```
buck test mode/dev-nosan caffe2/torch/fb/rendezvous:zeus_test
```
```
buck test mode/dev-nosan //caffe2/torch/fb/distributed/modules/tests:test_sharded_pairwise_attention_pooling -- test_single_trainer_multiple_pss
```
Differential Revision: D5524494
fbshipit-source-id: 50be58ec3c928621b0874b044ef4a1640534d8ef
