报错信息：

RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.FloatTensor [784, 512]], which is output 0 of TBackward, is at version 3; expected version 2 instead. Hint: enable anomaly detection to find the operation that failed to compute its gradient, with torch.autograd.set_detect_anomaly(True).

Traceback (most recent call last):
  File "E:\Anaconda\envs\torch_c_13\lib\site-packages\IPython\core\interactiveshell.py", line 3343, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-2-18bbaf295f9c>", line 1, in <module>
    runfile('E:/Code/AEs by PyTorch/AEsingle_train_test_temp.py', wdir='E:/Code/AEs by PyTorch')
  File "E:\SoftWare\PyCharm\PyCharm 2021.2.3\plugins\python\helpers\pydev\_pydev_bundle\pydev_umd.py", line 198, in runfile
    pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
  File "E:\SoftWare\PyCharm\PyCharm 2021.2.3\plugins\python\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "E:/Code/AEs by PyTorch/AEsingle_train_test_temp.py", line 205, in <module>
    train_ae_x_h2 = AEtrain(AEmodel2, train_ae_x_h1, 10, "AEmodel2")
  File "E:/Code/AEs by PyTorch/AEsingle_train_test_temp.py", line 95, in AEtrain
    loss.backward()
  File "E:\Anaconda\envs\torch_c_13\lib\site-packages\torch\tensor.py", line 195, in backward
    torch.autograd.backward(self, gradient, retain_graph, create_graph)
  File "E:\Anaconda\envs\torch_c_13\lib\site-packages\torch\autograd\__init__.py", line 99, in backward
    allow_unreachable=True)  # allow_unreachable flag
RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.FloatTensor [784, 512]], which is output 0 of TBackward, is at version 3; expected version 2 instead. Hint: enable anomaly detection to find the operation that failed to compute its gradient, with torch.autograd.set_detect_anomaly(True).

解决方法：

查看报错信息发现是loss.backward()部分报错，查阅多篇博客后找到解决方法如下

optimizer.zero_grad()
loss.backward(retain_graph=True)
optimizer.step()

将loss.backward()函数内的参数retain_graph值设置为True即可成功解决。

参考链接：

另，该问题可能在不同版本的pytorch中出现，可以参考以下链接的回答：

求pytorch大神解答，问题出在哪里 - 虎扑社区 (hupu.com)

也可能会因为代码中出现a=a.xxx这种赋值而报错，可改成b=a.xxx

【PyTorch】RuntimeError: one of the variables needed for gradient computation has been modified by an_ncc1995的博客-CSDN博客