add dygraph code for unstack (#27881)

python/paddle/fluid/layers/nn.py

@@ -10238,6 +10238,11 @@ def unstack(x, axis=0, num=None):
             y = paddle.unstack(x, axis=1)  # unstack with second axis, which results 3 tensors with shape=[2, 5]
 
     """
+    if in_dygraph_mode():
+        if num == None:
+            num = x.shape[axis]
+        return core.ops.unstack(x, num, 'axis', int(axis), 'num', num)
+
     helper = LayerHelper('unstack', **locals())
     if num is None:
         if axis is None or x.shape[axis] <= 0: