[remove fluid.layers.matmul] remove fluid.layers.matmul in example code (#48818)

* replace fluid.layers.matmul in fluid/io.py

* fix doc error in fluid.layers.nn.sampling_id

python/paddle/fluid/io.py

@@ -358,7 +358,7 @@ def save_vars(
                 data = fluid.layers.data(name="img", shape=[64, 784], append_batch_size=False)
                 w = paddle.create_parameter(shape=[784, 200], dtype='float32', name='fc_w')
                 b = paddle.create_parameter(shape=[200], dtype='float32', name='fc_b')
-                hidden_w = fluid.layers.matmul(x=data, y=w)
+                hidden_w = paddle.matmul(x=data, y=w)
                 hidden_b = fluid.layers.elementwise_add(hidden_w, b)
             place = fluid.CPUPlace()
             exe = fluid.Executor(place)
@@ -827,7 +827,7 @@ def load_vars(
                 data = fluid.layers.data(name="img", shape=[64, 784], append_batch_size=False)
                 w = paddle.create_parameter(shape=[784, 200], dtype='float32', name='fc_w')
                 b = paddle.create_parameter(shape=[200], dtype='float32', name='fc_b')
-                hidden_w = fluid.layers.matmul(x=data, y=w)
+                hidden_w = paddle.matmul(x=data, y=w)
                 hidden_b = fluid.layers.elementwise_add(hidden_w, b)
             place = fluid.CPUPlace()
             exe = fluid.Executor(place)
@@ -1592,7 +1592,7 @@ def load_inference_model(
                 data = fluid.layers.data(name="img", shape=[64, 784], append_batch_size=False)
                 w = paddle.create_parameter(shape=[784, 200], dtype='float32')
                 b = paddle.create_parameter(shape=[200], dtype='float32')
-                hidden_w = fluid.layers.matmul(x=data, y=w)
+                hidden_w = paddle.matmul(x=data, y=w)
                 hidden_b = fluid.layers.elementwise_add(hidden_w, b)
             place = fluid.CPUPlace()
             exe = fluid.Executor(place)