dygraph模式下，fluid.gradients 报错
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fluid.gradients这个api不适用于动态图。动态图下要先调y.backward()，再通过Variable.gradient()获取其梯度值。正确代码示例：
from __future__ import print_function
import paddle.fluid as fluid
import numpy as np
if __name__ == '__main__':
with fluid.dygraph.guard():
rng = np.random.RandomState(0)
inp_ = rng.uniform(-1, 1, [3, 2]).astype('float32')
w1_ = rng.uniform(-1, 1, [2, 5]).astype('float32')
w2_ = rng.uniform(-1, 1, [5, 4]).astype('float32')
inp = fluid.dygraph.base.to_variable(inp_)
w1 = fluid.dygraph.base.to_variable(w1_)
w2 = fluid.dygraph.base.to_variable(w2_)
inp.stop_gradient = False
w1.stop_gradient = False
w2.stop_gradient = False
y = fluid.layers.mul(fluid.layers.mul(inp, w1), w2)
y.backward()
print(w1.gradient())
print(w2.gradient())
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建立issue时，为快速解决问题，请您根据使用情况给出如下信息：
1）PaddlePaddle版本：1.6.1.post97 GPU版本
3）GPU：Tesla P40 cuDNN Version: 7.0,
4）系统环境：CentOS release 6.3 (Final)，Python3.6, 2018 x86_64 x86_64 x86_64 GNU/Linux
#!/usr/bin/env python
#-- coding=utf8 --
import paddle.fluid as fluid
import numpy as np
if name == 'main':
with fluid.dygraph.guard():
rng = np.random.RandomState(0)
inp_ = rng.uniform(-1, 1, [3, 2]).astype('float32')
w1_ = rng.uniform(-1, 1, [2, 5]).astype('float32')
w2_ = rng.uniform(-1, 1, [5, 4]).astype('float32')
inp = fluid.dygraph.base.to_variable(inp_)
w1 = fluid.dygraph.base.to_variable(w1_)
w2 = fluid.dygraph.base.to_variable(w2_)
inp.stop_gradient = False
w1.stop_gradient = False
w2.stop_gradient = False
y = fluid.layers.mul(fluid.layers.mul(inp, w1), w2)
x = [w1, w2]
dfdx_f1 =fluid.gradients(y, x)
print(dfdx_f1)
#######################
Traceback (most recent call last):
File "test_torch_dygraph.py", line 51, in
test_mul_grad_paddle_vs_torch()
File "test_torch_dygraph.py", line 37, in test_mul_grad_paddle_vs_torch
dfdx_f1 =fluid.gradients(f, x)
File "/home/users/liuyaping/local/python36/lib/python3.6/site-packages/paddle/fluid/backward.py", line 1350, in gradients
outs = calc_gradient(targets, inputs, target_gradients, no_grad_set)
File "/home/users/liuyaping/local/python36/lib/python3.6/site-packages/paddle/fluid/backward.py", line 1299, in calc_gradient
prog._sync_with_cpp()
File "/home/users/liuyaping/local/python36/lib/python3.6/site-packages/paddle/fluid/framework.py", line 4345, in _sync_with_cpp
block._sync_with_cpp()
File "/home/users/liuyaping/local/python36/lib/python3.6/site-packages/paddle/fluid/framework.py", line 2579, in _sync_with_cpp
op = Operator(self, op_desc)
File "/home/users/liuyaping/local/python36/lib/python3.6/site-packages/paddle/fluid/framework.py", line 1721, in init
"
typeto initialized an Operator can not be None.")ValueError:
typeto initialized an Operator can not be None.