[cleanup] nn.Pipe: deprecate Pipe when torch version >= 1.8.0 (#597)

fairscale/nn/pipe/pipe.py

@@ -20,6 +20,7 @@
 """The Pipe interface."""
 from collections import OrderedDict
 from typing import TYPE_CHECKING, Any, Iterable, List, Optional, Tuple, Union, cast
+import warnings
 
 import torch
 from torch import Tensor, nn
@@ -255,6 +256,14 @@ class Pipe(Module):
     ) -> None:
         super().__init__()
 
+        if torch.__version__.split(".")[:2] >= ["1", "8"]:
+            warnings.warn(
+                "fairscale.nn.Pipe has been upstreamed to PyTorch as torch.distributed.pipeline.sync.Pipe. "
+                "It is now deprecated and will be removed in a future version of fairscale. "
+                "The PyTorch API has minor changes. Please see https://pytorch.org/docs/stable/pipeline.html for details.",
+                DeprecationWarning,
+            )
+
         chunks = int(chunks)
         checkpoint = str(checkpoint)