add AvgPooling layer

mindspore/nn/layer/pooling.py

@@ -255,9 +255,26 @@ class AvgPool1d(_PoolNd):
     Examples:
         >>> pool = nn.AvgPool1d(kernel_size=3, strides=1)
         >>> x = Tensor(np.random.randint(0, 10, [1, 2, 4, 4]), mindspore.float32)
+        [[[[8. 8. 7. 4.]
+           [8. 4. 0. 9.]
+           [6. 4. 6. 1.]
+           [6. 8. 8. 5.]
+          [[4. 8. 5. 4.]
+           [8. 4. 7. 5.]
+           [3. 5. 3. 9.]
+           [7. 5. 4. 7.]]]]
         >>> output = pool(x)
         >>> output.shape()
+        (1, 2, 4, 2)
         >>> output
+        [[[[7.6640625 6.3320312]
+           [4.        4.3320312]
+           [5.3320312 3.6660156]
+           [7.3320312 7        ]
+          [[5.6640625 5.6640625]
+           [6.3320312 5.3320312]
+           [3.6660156 5.6640625]
+           [5.3320312 5.3320312]]]]
     """
 
     def __init__(self,