Fix Tensor's __repr__ for printing out grad (#3673)

* update check for Tensor's __repr__ with grad

* add test for repr with grad bugfix

test/test_tensor.py

@@ -334,6 +334,12 @@ class TestTinygrad(unittest.TestCase):
     with self.assertRaises(TypeError):
       _a = Tensor([3]) in [Tensor([3]), Tensor([4]), Tensor([5])]
 
+  def test_repr_with_grad(self):
+    a = Tensor([1])
+    b = Tensor([1])
+    c = (a + b).mean().backward()
+    print(c)
+
 @unittest.skipIf(CI and Device.DEFAULT in {"GPU", "CUDA", "METAL"}, "no GPU CI")
 class TestMoveTensor(unittest.TestCase):
   d0, d1 = f"{Device.DEFAULT}:0", f"{Device.DEFAULT}:1"

tinygrad/tensor.py

@@ -111,7 +111,7 @@ class Tensor:
     else:
       self.lazydata = data if data.device == device else data.copy_to_device(device)
 
-  def __repr__(self): return f"<Tensor {self.lazydata!r} on {self.device} with grad {(self.grad.lazydata if self.grad else None)!r}>"
+  def __repr__(self): return f"<Tensor {self.lazydata!r} on {self.device} with grad {(self.grad.lazydata if self.grad is not None else None)!r}>"
 
   # Python has a non moving GC, so this should be okay
   def __hash__(self): return id(self)