Fix broken THNN groups in conv functions

Author: apaszke

test/test_nn.py

@@ -1308,6 +1308,11 @@ def add_test(test):
         input_size=(2, 4, 6, 5),
         cudnn=True,
     ),
+    dict(
+        fullname='Conv2d_groups_thnn',
+        constructor=lambda: nn.Conv2d(4, 6, (3, 2), groups=2),
+        input_size=(2, 4, 6, 5),
+    ),
     dict(
         module_name='ConvTranspose2d',
         constructor_args=(3, 4, 3, (3, 2), 1, (1, 1)),

torch/nn/_functions/conv.py

@@ -149,6 +149,8 @@ def _thnn(self, fn_name, input, weight, *args):
             res = []
             for g in range(self.groups):
                 def group(tensor, dim=None):
+                    if tensor is None:
+                        return None
                     if dim is None:
                         dim = 0 if tensor.dim() == 1 else 1
                     n = tensor.size(dim) // self.groups
@@ -158,7 +160,8 @@ def group(tensor, dim=None):
                 grouped_args += [group(t) for t in args]
                 res.append(impl[fn_name](self, self._bufs[g], *grouped_args))
             if fn_name == 'grad_params':
-                return [torch.cat(t, 0) for t in zip(*res)]
+                return [torch.cat(t, 0) if t[0] is not None else None
+                                        for t in zip(*res)]
             else:
                 return torch.cat(res, 1)