code: def test_example_readme(self, device):
      x = Tensor.eye(3, device=device, requires_grad=True)
      y = Tensor([[2.0, 0, -2.0]], device=device, requires_grad=True)
      z = y.matmul(x).sum()
      z.backward()
  
      x.grad.numpy()  # dz/dx
      y.grad.numpy()  # dz/dy
  
      assert x.grad.device == device
      assert y.grad.device == device
  
file_path: tinygrad/test/external/external_test_example.py
imports: import unittest
  from tinygrad import Device
  from tinygrad.tensor import Tensor
  from tinygrad.helpers import getenv, CI
label_desc: Use image node_id 3 for calling the TestExample obj's underlying member method code with example usage: obj.test_example_readme(device) without return types