Distributed training of the network shown below gives this error, while without distributed works.
RuntimeError: one of the variables needed for gradient computation has been modified by an inplace operation: [torch.cuda.FloatTensor [12, 256, 1, 1]] is at version 2; expected version 1 instead.
[W python_anomaly_mode.cpp:60] Warning: Error detected in CudnnConvolutionBackward. Traceback of forward call that caused the error:
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/threading.py", line 890, in _bootstrap
    self._bootstrap_inner()
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/threading.py", line 926, in _bootstrap_inner
    self.run()
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/nn/parallel/parallel_apply.py", line 60, in _worker
    output = module(*input, **kwargs)
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/nn/modules/module.py", line 722, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/net.py", line 51, in forward
    out1 = self.conv2(out0.clone())
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/nn/modules/module.py", line 722, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/nn/modules/conv.py", line 419, in forward
    return self._conv_forward(input, self.weight)
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/nn/modules/conv.py", line 416, in _conv_forward
    self.padding, self.dilation, self.groups)
 (function print_stack)
Process Process-1:
Traceback (most recent call last):
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/home/e2r/Desktop/e2r/train.py", line 51, in init_process
    fn(rank, opt)
  File "/home/e2r/Desktop/e2r/train.py", line 194, in main_worker
    trainer.train()
  File "/home/e2r/Desktop/e2r//trainer.py", line 166, in train
    self.run_epoch()
  File "/home/e2r/Desktop/e2r/trainer.py", line 199, in run_epoch
    losses["loss"].backward()
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/tensor.py", line 185, in backward
    torch.autograd.backward(self, gradient, retain_graph, create_graph)
  File "/home/e2r/anaconda3/envs/e2r/lib/python3.7/site-packages/torch/autograd/__init__.py", line 127, 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.cuda.FloatTensor [12, 256, 1, 1]] is at version 2; expected version 1 instead. Hint: the backtrace further above shows the operation that failed to compute its gradient. The variable in question was changed in there or anywhere later. Good luck!


Enabling anamoly detection traces it back to a 1 x 1 convolution layer. I have tried many things to resolve it such as clone() before giving the layer as input and also having inplace=False in Relu layers. I still cannot resolve it. Here’s the network
class Net(nn.Module):
    def __init__(self, num_ch_enc, num_input_features, num_frames_to_predict_for=None, stride=1):
        super(Net, self).__init__()

        self.num_ch_enc = num_ch_enc
        self.num_input_features = num_input_features

        if num_frames_to_predict_for is None:
            num_frames_to_predict_for = num_input_features - 1
        self.num_frames_to_predict_for = num_frames_to_predict_for
        num_frames_to_predict_for_6 = int(6*self.num_frames_to_predict_for)

        self.squeeze = nn.Conv2d(self.num_ch_enc[-1], 256, 1)
        self.conv0 = nn.Conv2d(num_input_features * 256, 256, 3, stride, 1)
        self.conv1 = nn.Conv2d(256, 256, 3, stride, 1)
        self.conv2 = nn.Conv2d(256, num_frames_to_predict_for_6, 1, 1, 0)

        self.relu = nn.ReLU(inplace=False)

    def forward(self, input_features):
        last_features = [f[-1] for f in input_features]

        cat_features = [self.relu(self.squeeze(f)) for f in last_features]
        cat_features = torch.cat(cat_features, 1)

        out = cat_features
        out = self.conv0(out)
        out = self.relu(out)
        out = self.conv1(out)
        out = self.relu(out)
        out = self.conv1(out)
        out0 = self.relu(out)

        # gives inplace modification error here for multiprocessing for conv2 layer
        out1 = self.conv2(out0.clone())
        out2 = out1.mean(3).clone()
        out3 = out2.mean(2).clone()

        out4 = 0.01 * out3.clone().view(-1, self.num_frames_to_predict_for, 1, 6)

        axisangle = out4[..., :3]
        translation = out4[..., 3:]

        return out4[..., :3], out4[..., 3:]


Edit:
I tried to simplify the forward function as follows but I still get an error at the linear layer
def forward(self, x):
        x = x[-1][-1]

        x = self.pose3(x)

        x = x.view(-1, self.num_frames_to_predict_for * 6* 6* 20)
        
        # Same errror in the linear layer here
        x1 = self.linear(x).clone()

        x2 = 0.01 * x1.view(-1, self.num_frames_to_predict_for, 1, 6)

        return x2[..., :3], x2[..., 3:]