[minor] not creating a temp file on import (#641)

* [minor] not creating a temp file on import

* address review

* Revert "address review"

This reverts commit f65eb9bc7f7ea8829b1ac0a369ef9a3e6b56420a.
Co-authored-by: Min Xu <min.xu@acm.org>

fairscale/utils/testing.py

@@ -81,7 +81,7 @@ if torch.cuda.is_available():
     available_devices.append("cuda")
 
 
-_, filename_mpi = tempfile.mkstemp()
+filename_mpi: Optional[str] = None
 
 
 class IdentityLayer(Base):
@@ -118,7 +118,8 @@ def torch_version() -> Tuple[int, ...]:
     return tuple(int(n) for n in numbering)
 
 
-_smi_ver = None
+# Global variable to cache the results from the first nvidia-smi execution.
+_smi_ver: Optional[str] = None
 
 
 def torch_cuda_version(compiled: bool = False) -> Tuple[int, ...]:
@@ -303,7 +304,14 @@ def torch_spawn(world_sizes: Optional[List[int]] = None) -> Callable:
 
             error_queue = multiprocessing.get_context("spawn").SimpleQueue()
             if "OMPI_COMM_WORLD_RANK" in os.environ:
+                # TODO (Min): this global used to be assigned every time this file is imported.
+                #     I changed it to be assigned on first use. Should be the same, but I am not
+                #     sure this is used or is correct since different processes would have different
+                #     file names to init_process_group below. By initing, here, we don't leave
+                #     a temp file behind on importing time.
                 global filename_mpi
+                if filename_mpi is None:
+                    filename_mpi = tempfile.mkstemp()[1]
 
                 os.environ["RANK"] = os.environ["OMPI_COMM_WORLD_RANK"]
                 os.environ["WORLD_SIZE"] = os.environ["OMPI_COMM_WORLD_SIZE"]