dygraph support while_loop op (#22714)

* dygraph support while_loop op
test=develop

* refine assign
test=develop

python/paddle/fluid/layers/control_flow.py

@@ -18,7 +18,7 @@ from ..wrapped_decorator import signature_safe_contextmanager
 from .layer_function_generator import autodoc, templatedoc
 from .tensor import assign, cast, fill_constant
 from .. import core
-from ..framework import Program, Variable, Operator
+from ..framework import Program, Variable, Operator, in_dygraph_mode
 from ..layer_helper import LayerHelper, unique_name
 from .nn import logical_and, logical_not, logical_or
 from .utils import assert_same_structure, map_structure
@@ -999,6 +999,20 @@ def while_loop(cond, body, loop_vars, is_test=False, name=None):
             "the shape of the variable returned by cond should be [],"
             "but given shape as {0}.".format(list(pre_cond.shape)))
 
+    if in_dygraph_mode():
+        now_cond = pre_cond.numpy()[0]
+        while (now_cond):
+            output_vars = body(*loop_vars)
+            if not isinstance(output_vars, (list, tuple)):
+                output_vars = [output_vars]
+            if len(output_vars) != len(loop_vars):
+                raise ValueError(
+                    "body in while_loop should return the same arity "
+                    "(length and structure) and types as loop_vars")
+            now_cond = cond(*output_vars).numpy()[0]
+            loop_vars = output_vars
+        return loop_vars
+
     while_loop_block = While(pre_cond, is_test, name)
     with while_loop_block.block():
         output_vars = body(*loop_vars)

python/paddle/fluid/tests/unittests/test_layers.py

@@ -1393,6 +1393,41 @@ class TestLayer(LayerTest):
 
         self.assertTrue(np.allclose(static_ret, dy_ret_rlt))
 
+    def test_while_loop(self):
+        with self.static_graph():
+            i = layers.fill_constant(shape=[1], dtype='int64', value=0)
+            ten = layers.fill_constant(shape=[1], dtype='int64', value=10)
+
+            def cond(i):
+                return layers.less_than(i, ten)
+
+            def body(i):
+                return i + 1
+
+            out = layers.while_loop(cond, body, [i])
+            static_ret = self.get_static_graph_result(feed={}, fetch_list=out)
+
+        with self.dynamic_graph():
+            i = layers.fill_constant(shape=[1], dtype='int64', value=0)
+            ten = layers.fill_constant(shape=[1], dtype='int64', value=10)
+
+            def cond(i):
+                return layers.less_than(i, ten)
+
+            def body(i):
+                return i + 1
+
+            dy_ret = layers.while_loop(cond, body, [i])
+            with self.assertRaises(ValueError):
+                j = layers.fill_constant(shape=[1], dtype='int64', value=0)
+
+                def body2(i):
+                    return i + 1, i + 2
+
+                layers.while_loop(cond, body2, [j])
+
+        self.assertTrue(np.array_equal(static_ret[0], dy_ret[0].numpy()))
+
     def test_compare(self):
         value_a = np.arange(3)
         value_b = np.arange(3)