Fix example in log_loss doc (#22052)

* Fix example in log_loss doc, test=develop

* Use none for unknown dim, test=develop, test=document_fix

python/paddle/fluid/layers/nn.py

@@ -11964,8 +11964,8 @@ def log_loss(input, label, epsilon=1e-4, name=None):
         .. code-block:: python
 
           import paddle.fluid as fluid
-          label = fluid.data(name='label', shape=[-1, 1], dtype='int64')
-          prob = fluid.data(name='prob', shape=[-1, 10], dtype='float32')
+          label = fluid.data(name='label', shape=[None, 1], dtype='float32')
+          prob = fluid.data(name='prob', shape=[None, 1], dtype='float32')
           cost = fluid.layers.log_loss(input=prob, label=label)
     """
     helper = LayerHelper('log_loss', **locals())