changed tensor.grad to have list-in, list-out behavior

theano/tensor/tensor_grad.py

@@ -268,7 +268,9 @@ def grad(cost, wrt, g_cost=None, consider_constant=None, warn_type=False,
     # such subtle cases can be fixed by a more careful implementation of the
     # gradient, but for now Theano needs to throw an exception, and make the
     # user aware that it does not know how to compute that gradient
-    if not isinstance(wrt, (list, tuple)):
+    using_list = isinstance(wrt, list)
+    using_tuple = isinstance(list, tuple)
+    if not (using_list or using_tuple):
         wrt = [wrt]
     ret = []
     for p in wrt:
@@ -292,8 +294,15 @@ def grad(cost, wrt, g_cost=None, consider_constant=None, warn_type=False,
             ret.append(zeros_like(p))
 
     if len(ret) == 1:
-        return ret[0]
+        if using_list:
+            return ret
+        elif using_tuple:
+            return tuple(ret)
+        else:
+            return ret[0]
     else:
+        if using_tuple:
+            return tuple(ret)
         return ret
 
 class numeric_grad: