add example of defining new autograd function

Author: jcjohnson

new_function.py

@@ -0,0 +1,19 @@
+import torch
+from torch.autograd import Variable
+
+
+class MulTwo(torch.autograd.Function):
+  def forward(self, input):
+    return 2.0 * input
+
+  def backward(self, grad_output):
+    return 2.0 * grad_output
+
+
+x = Variable(torch.randn(3, 4), requires_grad=True)
+y = MulTwo()(x)
+s = y.sum()
+
+s.backward()
+print(x.grad.data)
+