[torch/multiprocessing] Use multiprocessing.reduction.register ForkingPickler.register to register custom tensor and storage reductions (#135030)

Right now `multiprocessing.reduction.register()` is simply an alias to `multiprocessing.reduction.ForkingPickler.register()`
https://github.com/python/cpython/blame/main/Lib/multiprocessing/reduction.py#L56, but the top-level `register()` function exposes less of the internal details of `multiprocessing.reduction` module.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/135030
Approved by: https://github.com/albanD

torch/multiprocessing/reductions.py

@@ -2,7 +2,7 @@
 import multiprocessing
 import os
 import threading
-from multiprocessing.reduction import ForkingPickler
+from multiprocessing import reduction
 from multiprocessing.util import register_after_fork
 from typing import Union
 
@@ -626,22 +626,22 @@ def reduce_storage(storage):
 
 
 def init_reductions():
-    ForkingPickler.register(torch.cuda.Event, reduce_event)
+    reduction.register(torch.cuda.Event, reduce_event)
 
     for t in torch._storage_classes:
         if t.__name__ == "UntypedStorage":
-            ForkingPickler.register(t, reduce_storage)
+            reduction.register(t, reduce_storage)
         else:
-            ForkingPickler.register(t, reduce_typed_storage_child)
+            reduction.register(t, reduce_typed_storage_child)
 
-    ForkingPickler.register(torch.storage.TypedStorage, reduce_typed_storage)
+    reduction.register(torch.storage.TypedStorage, reduce_typed_storage)
 
     for t in torch._tensor_classes:
-        ForkingPickler.register(t, reduce_tensor)
+        reduction.register(t, reduce_tensor)
 
     # TODO: Maybe this should be in tensor_classes? :)
-    ForkingPickler.register(torch.Tensor, reduce_tensor)
+    reduction.register(torch.Tensor, reduce_tensor)
 
     from torch.nn.parameter import Parameter
 
-    ForkingPickler.register(Parameter, reduce_tensor)
+    reduction.register(Parameter, reduce_tensor)