[feat] add sha1_store get function (#1027)

Co-authored-by: Min Xu <min.xu.public@gmail.com>

fairscale/experimental/wgit/sha1_store.py

@@ -182,7 +182,15 @@ class SHA1_Store:
             (Tensor or OrderedDict):
                 In-memory object.
         """
-        raise NotImplementedError()
+        path = self._sha1_to_dir(sha1).joinpath(sha1)
+        if not path.exists():
+            # This is potentially valid case for the caller, we need to inform the
+            # the caller about it.
+            raise ValueError(f"Try to get SHA1 {sha1} but it is not found")
+        # Directly return the object after loading it. This could be throw an
+        # exception but that indicates some internal error since we should never
+        # have stored the (invalid) object in the first place with the add() API.
+        return torch.load(path)
 
     def delete(self, sha1: str) -> None:
         """Delete a SHA1

tests/experimental/wgit/test_sha1_store.py

@@ -11,6 +11,7 @@ import pytest
 import torch
 from torch import nn
 
+from fair_dev.testing.testing import objects_are_equal
 from fairscale.experimental.wgit.sha1_store import SHA1_Store
 from fairscale.internal import torch_version
 
@@ -111,3 +112,30 @@ def test_sha1_add_tensor(sha1_store):
         # torch 1.8 LTS doesn't produce deterministic checkpoint file from fixed tensors/state_dict.
         key = "71df4069a03a766eacf9f03eea50968e87eae9f8"
         assert key in json_dict.keys() and json_dict[key] == 1, json_dict
+
+
+def test_sha1_get(sha1_store):
+    os.chdir(PARENT_DIR)
+
+    # Add a file, a state dict and a tensor.
+    file = "test_get.pt"
+    torch.save(nn.Linear(100, 100).state_dict(), file)
+    state_dict = nn.Sequential(nn.Linear(10, 10), nn.Linear(10, 20)).state_dict()
+    tensor = torch.ones(20, 30)
+
+    # Check that we can get them back.
+    file_sha1 = sha1_store.add(file)
+    sd = sha1_store.get(file_sha1)
+    assert objects_are_equal(sd, torch.load(file))
+
+    sd_sha1 = sha1_store.add(state_dict)
+    sd = sha1_store.get(sd_sha1)
+    assert objects_are_equal(sd, state_dict)
+
+    tensor_sha1 = sha1_store.add(tensor)
+    tensor_got = sha1_store.get(tensor_sha1)
+    assert objects_are_equal(tensor_got, tensor)
+
+    # Make sure invalid sha1 cause exceptions.
+    with pytest.raises(ValueError):
+        sha1_store.get(tensor_sha1[:-1])