docs: Update docstring to include link for extending torch.autograd

Author: harshaljanjani

beginner_source/examples_autograd/polynomial_custom_function.py

@@ -35,8 +35,9 @@ def forward(ctx, input):
         a Tensor containing the output. ctx is a context object that can be used
         to stash information for backward computation. You can cache tensors for
         use in the backward pass using the ``ctx.save_for_backward`` method. Other
-        objects can be stored directly as attributes on the ctx object, such as 
-        ctx.my_object = my_object.
+        objects can be stored directly as attributes on the ctx object, such as
+        ctx.my_object = my_object. Check out `Extending torch.autograd <https://docs.pytorch.org/docs/stable/notes/extending.html#extending-torch-autograd>`_
+        for further details.
         """
         ctx.save_for_backward(input)
         return 0.5 * (5 * input ** 3 - 3 * input)