Fixes #17304 Flaky Test -> test_higher_order_grad.test_tanh (#17321)

* use failed seed and verify first order

* replace grad_op with equivalent expression

* remove fixed seed for tanh

* add relax tolerance for tanh first order

tests/python/unittest/test_higher_order_grad.py

@@ -121,14 +121,15 @@ def tanh(x):
         return nd.tanh(x)
 
     def grad_op(x):
-        return 1 / nd.cosh(x)**2
+        return 1 - tanh(x)**2
 
     def grad_grad_op(x):
         return -2 * tanh(x) * grad_op(x)
 
     for dim in range(1, 5):
         shape = rand_shape_nd(dim)
         array = random_arrays(shape)
+        check_nth_order_unary(array, tanh, grad_op, 1, rtol=1e-6, atol=1e-6)
         check_second_order_unary(
             array, tanh, grad_grad_op, rtol=1e-6, atol=1e-6)