Use Tensor.to(torch.bool) instead of Tensor.bool()

Tensor.bool() was introduced in PyTorch 1.2.0.

tests/test_bugs.py

@@ -119,4 +119,4 @@ def forward(self, x):
     y = model(x)
     y.norm().backward()
 
-    assert y.bool().tolist() == x.grad.bool().tolist()
+    assert y.to(torch.bool).tolist() == x.grad.to(torch.bool).tolist()