Change init method from fork-server to spawn (#2427)

Summary:
Pull Request resolved: #2427

It seems AMD GPU runtime doesn't quite work with forkserver for some reasons that we still need to debug. Basically hipMalloc will fail for subprocess. So just change it to spawn - I think it should be fine?

Differential Revision: D63311340

torchrec/distributed/test_utils/multi_process.py

@@ -24,6 +24,9 @@
 )
 
 
+_MP_INIT_MODE = "forkserver" if torch.version.hip else "spawn"
+
+
 class MultiProcessContext:
     def __init__(
         self,
@@ -126,7 +129,7 @@ def _run_multi_process_test(
         # pyre-ignore
         **kwargs,
     ) -> None:
-        ctx = multiprocessing.get_context("forkserver")
+        ctx = multiprocessing.get_context(_MP_INIT_MODE)
         processes = []
         for rank in range(world_size):
             kwargs["rank"] = rank
@@ -152,7 +155,7 @@ def _run_multi_process_test_per_rank(
         world_size: int,
         kwargs_per_rank: List[Dict[str, Any]],
     ) -> None:
-        ctx = multiprocessing.get_context("forkserver")
+        ctx = multiprocessing.get_context(_MP_INIT_MODE)
         processes = []
         for rank in range(world_size):
             kwargs = {}