[enhancement] Implement tensor.cpu() in static graph (#55921)

* [enhancement] Implement tensor.cpu() in static graph

* [enhancement] Implement tensor.cpu() in static graph

* [dy2static] add unitest cases for tensor.cpu()

* [dy2static] add unitest cases for tensor.cpu() - run only on paddle compiled with cuda mode

* [dy2static]Refine the format of the api-doc for Variable.cpu().

python/paddle/fluid/layers/math_op_patch.py

@@ -136,11 +136,41 @@ def monkey_patch_variable():
     @static_only
     def cpu(self):
         """
-        Variable should not have cpu() and cuda() interface.
-        But this interface can greatly facilitate dy2static.
-        We do nothing here.
+        In dy2static, Variable also needs cpu() and cuda() interface.
+        But, the underneath operator has only forward op but not backward one.
+
+        Returns:
+            The tensor which has copied to cpu place.
+
+        Examples:
+            In Static Graph Mode:
+
+            .. code-block:: python
+                import paddle
+                paddle.enable_static()
+
+                x = paddle.static.data(name="x", shape=[2,2], dtype='float32')
+                y = x.cpu()
         """
-        return self
+        block = current_block(self)
+        tmp_name = unique_tmp_name()
+        output = block.create_var(
+            name=tmp_name,
+            dtype=self.dtype,
+            shape=self.shape,
+            type=self.type,
+            persistable=False,
+            stop_gradient=True,
+        )
+        # 0 means cpu place, see paddle/fluid/operators/memcpy_op.h
+        attrs = {'dst_place_type': 0}
+        block.append_op(
+            type='memcpy',
+            inputs={'X': [self]},
+            outputs={'Out': [output]},
+            attrs=attrs,
+        )
+        return output
 
     @static_only
     def cuda(self):

test/dygraph_to_static/test_tensor_memcpy_on_cpu.py

+# Copyright (c) 2023 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+
+import numpy as np
+
+import paddle
+
+
+@paddle.jit.to_static
+def tensor_copy_to_cpu(x):
+    x = paddle.to_tensor(x)
+    y = x.cpu()
+    return y
+
+
+class TestTensorCopyToCpuOnDefaultCPU(unittest.TestCase):
+    def _run(self, to_static):
+        paddle.jit.enable_to_static(to_static)
+        x1 = paddle.ones([1, 2, 3])
+        x2 = tensor_copy_to_cpu(x1)
+        return x1.place, x2.place, x2.numpy()
+
+    def test_tensor_cpu_on_default_cpu(self):
+        paddle.fluid.framework._set_expected_place(paddle.CPUPlace())
+        dygraph_x1_place, dygraph_place, dygraph_res = self._run(
+            to_static=False
+        )
+        static_x1_place, static_place, static_res = self._run(to_static=True)
+        np.testing.assert_allclose(dygraph_res, static_res, rtol=1e-05)
+        self.assertTrue(dygraph_x1_place.is_cpu_place())
+        self.assertTrue(static_x1_place.is_cpu_place())
+        self.assertTrue(dygraph_place.is_cpu_place())
+        self.assertTrue(static_place.is_cpu_place())
+
+
+if __name__ == '__main__':
+    unittest.main()

test/dygraph_to_static/test_tensor_memcpy_on_gpu.py

+# Copyright (c) 2023 PaddlePaddle Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+import unittest
+
+import numpy as np
+
+import paddle
+
+
+@paddle.jit.to_static
+def tensor_copy_to_cpu(x):
+    x = paddle.to_tensor(x)
+    y = x.cpu()
+    return y
+
+
+class TestTensorCopyToCpuOnDefaultGPU(unittest.TestCase):
+    def _run(self, to_static):
+        paddle.jit.enable_to_static(to_static)
+        x1 = paddle.ones([1, 2, 3])
+        x2 = tensor_copy_to_cpu(x1)
+        return x1.place, x2.place, x2.numpy()
+
+    def test_tensor_cpu_on_default_gpu(self):
+        if paddle.fluid.is_compiled_with_cuda():
+            place = paddle.CUDAPlace(
+                int(os.environ.get('FLAGS_selected_gpus', 0))
+            )
+        else:
+            return
+        paddle.fluid.framework._set_expected_place(place)
+        dygraph_x1_place, dygraph_place, dygraph_res = self._run(
+            to_static=False
+        )
+        static_x1_place, static_place, static_res = self._run(to_static=True)
+        np.testing.assert_allclose(dygraph_res, static_res, rtol=1e-05)
+        self.assertTrue(dygraph_x1_place.is_gpu_place())
+        self.assertTrue(static_x1_place.is_gpu_place())
+        self.assertTrue(dygraph_place.is_cpu_place())
+        self.assertTrue(static_place.is_cpu_place())
+
+
+if __name__ == '__main__':
+    unittest.main()