added support class update_gradient_via_grad

grad.py

@@ -144,6 +144,35 @@ def grad(cost, param=None, cost_grad = 1.0):
     else:
         return rval(param)
 
+class update_gradient_via_grad:
+    """Inherit from this class to add a convenient self.update_gradient function"""
+
+    def update_gradient(self, grad_d):
+        """Call self.grad() and add the result to grad_d
+
+        This function is called by grad.Grad.bprop() to construct a symbolic gradient graph.
+
+        self.grad is called like this:
+
+            self.grad(*(self.inputs + [grad_d[output] for output in self.outputs]))
+
+        In general, grad() should return a list of PythonR instances whose
+        length matches that of self.inputs, and whose elements are the
+        gradients of self.inputs.
+
+        There is a (but often used) special feature in place to automatically
+        wrap the return value of grad() in a list if it is a PythonR instance
+        and the op is unary.  This makes many grad implementations a little
+        cuter.
+
+        """
+        inputgs = self.grad(*(self.inputs + [grad_d[output] for output in self.outputs]))
+        if len(self.inputs) == 1 and isinstance(inputgs, gof.PythonR):
+            inputgs = [inputgs]
+        else:
+            assert len(inputgs) == len(self.inputs)
+        for input, inputg in zip(self.inputs, inputgs):
+            grad_d.add(input, inputg)
 
 #
 # UNIT TEST