Refine assign layer

Give assign layer's second parameter 'output' a default value: None. If
it is None, the output variable will be created inside the layer.

python/paddle/fluid/layers/tensor.py

@@ -234,7 +234,7 @@ def sums(input, out=None):
     return out
 
 
-def assign(input, output):
+def assign(input, output=None):
     """
     **Assign**
 
@@ -242,7 +242,7 @@ def assign(input, output):
 
     Args:
         input(Variable|numpy.ndarray): The source variable
-        output(Variable): The destination variable
+        output(Variable|None): The destination variable
 
     Returns:
         Variable: The destination variable that was supplied as the *output*.
@@ -255,6 +255,8 @@ def assign(input, output):
           fluid.layers.assign(hidden, out)
     """
     helper = LayerHelper('assign', **locals())
+    if output is None:
+        output = helper.create_tmp_variable(dtype=input.dtype)
     if isinstance(input, Variable):
         helper.append_op(
             type='assign', inputs={'X': [input]}, outputs={'Out': [output]})