Nets now can be loaded even if device=None (#876)

In #600, we introduced the option to set device=None, which means that
skorch should not move any data to any device. However, this setting
introduced a bug when trying to load the net, as that code didn't
explicitly deal with device=None. With this PR, the bug is fixed.

Implementation

If device=None, we really have no way of knowing what device to map the
parameters to. The most reasonable thing to do is to use the fallback,
which is 'cpu'.

CHANGES.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fix a bug in `SliceDataset` that prevented it to be used with `to_numpy` (#858)
+- Fix a bug that occurred when loading a net that has device set to None
 
 ## [0.11.0] - 2021-10-11
 

skorch/net.py

@@ -2413,6 +2413,15 @@ def _check_device(self, requested_device, map_device):
         return the map device if it differs from the requested device
         along with a warning.
         """
+        if requested_device is None:
+            # user has set net.device=None, we don't know the type, use fallback
+            msg = (
+                f"Setting self.device = {map_device} since the requested device "
+                f"was not specified"
+            )
+            warnings.warn(msg, DeviceWarning)
+            return map_device
+
         type_1 = torch.device(requested_device)
         type_2 = torch.device(map_device)
         if type_1 != type_2:

skorch/tests/test_net.py

@@ -383,6 +383,22 @@ def test_pickle_save_load(self, net_pickleable, data, tmpdir):
         score_after = accuracy_score(y, net_new.predict(X))
         assert np.isclose(score_after, score_before)
 
+    def test_pickle_save_load_device_is_none(self, net_pickleable):
+        # It is legal to set device=None, but in that case we cannot know what
+        # device was meant, so we should fall back to CPU.
+        from skorch.exceptions import DeviceWarning
+
+        net_pickleable.set_params(device=None)
+        msg = (
+            f"Setting self.device = cpu since the requested device "
+            f"was not specified"
+        )
+        with pytest.warns(DeviceWarning, match=msg):
+            net_loaded = pickle.loads(pickle.dumps(net_pickleable))
+
+        params = net_loaded.get_all_learnable_params()
+        assert all(param.device.type == 'cpu' for _, param in params)
+
     def train_picklable_cuda_net(self, net_pickleable, data):
         X, y = data
         w = torch.FloatTensor([1.] * int(y.max() + 1)).to('cuda')