add maxunpool2d in __all__ (#37698)

* add maxunpool2d in __all__

* fix MaxUnPool2D example

python/paddle/nn/__init__.py

@@ -295,5 +295,6 @@ __all__ = [     #noqa
            'ELU',
            'ReLU6',
            'LayerDict',
-           'ZeroPad2D'
+           'ZeroPad2D',
+           'MaxUnPool2D',
 ]

python/paddle/nn/layer/pooling.py

@@ -1181,7 +1181,7 @@ class MaxUnPool2D(Layer):
         pool_out, indices = F.max_pool2d(data, kernel_size=2, stride=2, padding=0, return_mask=True)
         # pool_out shape: [1, 1, 3, 3],  indices shape: [1, 1, 3, 3]
         Unpool2D = paddle.nn.MaxUnPool2D(kernel_size=2, padding=0)
-        unpool_out = UnPool2D(pool_out, indices)
+        unpool_out = Unpool2D(pool_out, indices)
         # unpool_out shape: [1, 1, 6, 6]
 
     """