code: def test_fori_loop(self, backend):
      with fastmath.use_backend(backend):
          res = fastmath.fori_loop(2, 5, lambda i, x: x + i, 1)
          self.assertEqual(res, 1 + 2 + 3 + 4)
  
imports: import collections
  from absl.testing import parameterized
  import gin
  import jax.numpy
  import numpy
  from tensorflow import test
  from trax import fastmath
label_desc: Use image node_id 6 for calling the BackendTest obj's underlying member method code with example usage: obj.test_fori_loop(backend) without return types