set pg name based on ranks (pytorch#166182)

Summary:
- in torchft we have multiple default pg's, 1 for each task group
- for flight recorder to work, each of these need to have a different name, so entries can be matched
- change the `init_process_group` api to optionally take a list of ranks. if provided, we use the hash of the ranks as the name of the pg. for torchft, we'll pass global ranks here so the default pg have a different name on each task group

Pull Request resolved: pytorch#166182
Approved by: https://github.com/fduwjj

Author: tushar00jain

torch/distributed/distributed_c10d.py

@@ -1583,6 +1583,7 @@ def init_process_group(
     group_name: str = "",
     pg_options: Optional[Any] = None,
     device_id: Optional[Union[torch.device, int]] = None,
+    _ranks: Optional[list[int]] = None,
 ) -> None:
     """
     Initialize the default distributed process group.
@@ -1657,6 +1658,8 @@ def init_process_group(
             want to know NCCL initialization error early, you can also use this
             field. If an `int` is provided, the API assumes that the accelerator
             type at compile time will be used.
+        _ranks: The ranks in the process group. If provided, the process
+               group name will be the hash of all the ranks in the group.
 
     .. note:: To enable ``backend == Backend.MPI``, PyTorch needs to be built from source
         on a system that supports MPI.
@@ -1761,7 +1764,10 @@ def init_process_group(
     internals of c10d. This means we can ignore the value
     they provide as it not exposed in a public way.
     """
-    group_name = _process_group_name([], use_hashed_name=False)
+    if _ranks is None or len(_ranks) == 0:
+        group_name = _process_group_name([], use_hashed_name=False)
+    else:
+        group_name = _process_group_name(_ranks, use_hashed_name=True)
     if backend == Backend.MPI:
         if world_size != -1 or rank != -1:
             warnings.warn(