Add fast path for dropout when p == 0 (#29553)

* add fast path for p == 0 in dropout

* add ut

python/paddle/fluid/dygraph/nn.py

@@ -1476,6 +1476,9 @@ class Dropout(layers.Layer):
         self._is_test = is_test
 
     def forward(self, input):
+        # fast return for p == 0
+        if self._dropout_prob == 0:
+            return input
         prog = default_main_program()
         if (self._seed is None or self._seed == 0) and prog.random_seed != 0:
             self._seed = prog.random_seed

python/paddle/fluid/layers/nn.py

@@ -1007,6 +1007,9 @@ def dropout(x,
             x = fluid.data(name="data", shape=[None, 32, 32], dtype="float32")
             dropped = fluid.layers.dropout(x, dropout_prob=0.5)
     """
+    # fast return for p == 0
+    if dropout_prob == 0:
+        return x
 
     def get_attrs(prog, dropout_prob, is_test, seed):
         if (seed is None or seed == 0) and prog.random_seed != 0:

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

@@ -302,13 +302,16 @@ class TestDropoutFAPI(unittest.TestCase):
                 training=False,
                 mode='downscale_in_infer')
             res10 = paddle.nn.functional.dropout(x=input, p=1., training=True)
+            res11 = paddle.fluid.layers.dropout(x=input, dropout_prob=0.)
 
             in_np = np.random.random([40, 40]).astype("float32")
             res_np = in_np
             res_np2 = np.zeros_like(in_np)
 
             exe = fluid.Executor(place)
-            res_list = [res1, res2, res3, res4, res5, res6, res7, res8, res9]
+            res_list = [
+                res1, res2, res3, res4, res5, res6, res7, res8, res9, res11
+            ]
             for res in res_list:
                 fetches = exe.run(fluid.default_main_program(),
                                   feed={"input": in_np},
@@ -383,8 +386,12 @@ class TestDropoutFAPI(unittest.TestCase):
                     mode='downscale_in_infer')
                 res10 = paddle.nn.functional.dropout(
                     x=input, p=1., training=True)
+                dropout = paddle.fluid.dygraph.Dropout(p=0, )
+                res11 = dropout(input)
 
-            res_list = [res1, res2, res3, res4, res5, res6, res7, res8, res9]
+            res_list = [
+                res1, res2, res3, res4, res5, res6, res7, res8, res9, res11
+            ]
             for res in res_list:
                 self.assertTrue(np.allclose(res.numpy(), res_np))
             self.assertTrue(np.allclose(res10.numpy(), res_np2))

python/paddle/nn/functional/common.py

@@ -887,6 +887,10 @@ def dropout(x,
             print(y_01)
 
     """
+    # fast return for p == 0
+    if p == 0:
+        return x
+
     if not isinstance(p, (float, int)):
         raise TypeError("p argument should be a number")
     if p < 0 or p > 1: