[MXNET-978] n-th order gradient test support. (apache#15611)

* n-th order grad test support

* use check_nth_order_unary for second order check

* add docstring to check_nth_order_unary

* retrigger CI

* add assertions for requirements of orders

* fix assert condition

* retrigger CI

tests/python/unittest/test_higher_order_grad.py

@@ -31,10 +31,16 @@ def sin(x):
     def grad_grad_op(x):
         return -nd.sin(x)
 
+    def grad_grad_grad_op(x):
+        return -nd.cos(x)
+
     for dim in range(1, 5):
         shape = rand_shape_nd(dim)
         array = random_arrays(shape)
         check_second_order_unary(array, sin, grad_grad_op)
+        # TODO(kshitij12345): Remove
+        check_nth_order_unary(array, sin,
+                              [grad_grad_op, grad_grad_grad_op], [2, 3])
 
 
 @with_seed()
@@ -45,10 +51,16 @@ def cos(x):
     def grad_grad_op(x):
         return -nd.cos(x)
 
+    def grad_grad_grad_op(x):
+        return nd.sin(x)
+
     for dim in range(1, 5):
         shape = rand_shape_nd(dim)
         array = random_arrays(shape)
         check_second_order_unary(array, cos, grad_grad_op)
+        # TODO(kshitij12345): Remove
+        check_nth_order_unary(array, cos,
+                              [grad_grad_op, grad_grad_grad_op], [2, 3])
 
 
 @with_seed()
@@ -178,13 +190,18 @@ def test_log():
     def log(x):
         return nd.log(x)
 
+    def grad_op(x):
+        return 1/x
+
     def grad_grad_op(x):
         return -1/(x**2)
 
     for dim in range(1, 5):
         shape = rand_shape_nd(dim)
         array = random_arrays(shape)
         check_second_order_unary(array, log, grad_grad_op)
+        # TODO(kshitij12345): Remove
+        check_nth_order_unary(array, log, [grad_op, grad_grad_op], [1, 2])
 
 
 @with_seed()
@@ -288,6 +305,9 @@ def grad_grad_op(x):
         shape = rand_shape_nd(dim)
         array = random_arrays(shape)
         check_second_order_unary(array, sigmoid, grad_grad_op)
+        # TODO(kshitij12345): Remove
+        check_nth_order_unary(array, sigmoid, [grad_op, grad_grad_op], [1, 2])
+        check_nth_order_unary(array, sigmoid, grad_grad_op, 2)
 
 
 @with_seed()
@@ -331,28 +351,77 @@ def grad_grad_op(x):
 
 
 def check_second_order_unary(x, op, grad_grad_op, rtol=None, atol=None):
+    check_nth_order_unary(x, op, grad_grad_op, 2, rtol, atol)
+
+
+def check_nth_order_unary(x, op, grad_ops, orders, rtol=None, atol=None):
+    """Assert n-th order autograd gradient against expected gradient.
+
+    Multiple order of gradients can be checked by passing list of
+    function computing the particular order gradient and passing the
+    corresponding list of order.
+
+    Note
+    ----
+    1. Orders should always be monotonically increasing.
+    2. Elements of grads_ops should correspond to elements of orders
+    i.e. grads_op = [grad_op, grad_grad_grad_op] should be passed with
+         orders = [1, 3]
+
+    Parameters
+    ----------
+    x : mxnet.NDArray
+        Input Array.
+    op : Callable
+        Operation to perform on Input Array.
+    grad_ops : Callable or List of Callable
+        Function to compute and assert gradient of given order.
+    orders : int or List of int
+        Order/s to assert expected and computed gradients.
+
+    Returns
+    -------
+    None
+
+    """
+    if isinstance(orders, int):
+        orders = [orders]
+        grad_ops = [grad_ops]
+
+    assert all(i < j for i, j in zip(orders[0:-1], orders[1:])), \
+        "orders should be monotonically increasing"
+    assert len(set(orders)) == len(orders), \
+        "orders should have unique elements"
+    highest_order = max(orders)
+
     x = nd.array(x)
-    grad_grad_x = grad_grad_op(x)
     x.attach_grad()
 
-    # Manual head_grads.
-    y_grad = nd.random.normal(shape=x.shape)
-    head_grad_grads = nd.random.normal(shape=x.shape)
+    expected_grads = [grad_op(x) for grad_op in grad_ops]
+    computed_grads = []
+    head_grads = []
 
     # Perform compute.
     with autograd.record():
         y = op(x)
-        x_grad = autograd.grad(heads=y, variables=x, head_grads=y_grad,
-                               create_graph=True, retain_graph=True)[0]
-    x_grad.backward(head_grad_grads)
-
-    # Compute expected values.
-    expected_grad_grad = grad_grad_x.asnumpy() * head_grad_grads.asnumpy() * \
-        y_grad.asnumpy()
-
-    # Validate the gradients.
-    assert_almost_equal(expected_grad_grad,
-                        x.grad.asnumpy(), rtol=rtol, atol=atol)
+        for current_order in range(1, highest_order+1):
+            head_grad = nd.random.normal(shape=x.shape)
+            y = autograd.grad(heads=y, variables=x, head_grads=head_grad,
+                              create_graph=True, retain_graph=True)[0]
+            if current_order in orders:
+                computed_grads.append(y)
+            head_grads.append(head_grad)
+
+    # Validate all the gradients.
+    for order, grad, computed_grad in \
+            zip(orders, expected_grads, computed_grads):
+        # Compute expected values.
+        expected_grad = grad.asnumpy()
+        for head_grad in head_grads[:order]:
+            expected_grad *= head_grad.asnumpy()
+
+        assert_almost_equal(
+            expected_grad, computed_grad.asnumpy(), rtol=rtol, atol=atol)
 
 
 if __name__ == '__main__':