Fix torch.autograd.backward inputs validation (#150975)

- Fixes #150883
- Fixes #70504

This is my first PR to pytorch, so please tell me if I'm forgetting anything.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/150975
Approved by: https://github.com/soulitzer

test/test_autograd.py

@@ -2507,6 +2507,12 @@ class TestAutograd(TestCase):
             lambda: torch.autograd.backward(fn(), gradient, inputs=[]),
         )
 
+    def test_backward_with_scalar_input(self):
+        x = torch.randn([], dtype=torch.double, requires_grad=True)
+        out = x**2
+        out.backward(inputs=x)
+        self.assertEqual(x.grad, 2 * x)
+
     def test_backward_with_nonleaf_inputs(self):
         x = torch.randn(2, 2, dtype=torch.double, requires_grad=True)
         x_nonleaf = x * 1