Hi,
I tried to follow this tutorial (https://pytorch.org/tutorials/advanced/super_resolution_with_caffe2.html 4) on converting a pytorch model to onnx for mobile usage. I used the current nightly build (shipped via pip). It worked fine with a simple model like
class RegNet(torch.nn.Module):
    def __init__(self):
        super().__init__()
        self.features = torch.nn.Sequential(
            torch.nn.Conv2d(3, 6, 5),
            torch.nn.ReLU(),
            torch.nn.MaxPool2d(2, 2),
            torch.nn.Conv2d(6, 16, 5),
            torch.nn.ReLU(),
            torch.nn.MaxPool2d(2, 2)
        )
        self.classifier = torch.nn.Sequential(
            torch.nn.Linear(16 * 5 * 5, 120),
            torch.nn.BatchNorm1d(120),
            torch.nn.ReLU(),
            torch.nn.Dropout(0.25),
            torch.nn.Linear(120, 84),
            torch.nn.BatchNorm1d(84),
            torch.nn.ReLU(),
            torch.nn.Linear(84, 1)
        )

    def forward(self, x):
        x = self.features(x)
        x = x.view(x.size(0), -1)
        x = self.classifier(x)
        return x

but did not work with a more complicated model like the U-Net-Generator from this repo (https://github.com/junyanz/pytorch-CycleGAN-and-pix2pix/blob/master/models/networks.py 4) (line 237).
My approach was both times the same:
torch.onnx.export(net, torch.rand(1, 3, 480, 480), "tmp.onxx", export_params=True) (didn’t work with torch.onnx._export(...) either.
The error message is quite long:
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/schock/anaconda3/envs/pytorch_nightly/lib/python3.6/site-packages/torch/onnx/__init__.py", line 22, in _export
    return utils._export(*args, **kwargs)
  File "/home/schock/anaconda3/envs/pytorch_nightly/lib/python3.6/site-packages/torch/onnx/utils.py", line 286, in _export
    proto, export_map = graph.export(params, _onnx_opset_version, defer_weight_export, operator_export_type)
RuntimeError: block->inputs().size() >= num_initializers ASSERT FAILED at /pytorch/torch/csrc/jit/export.cpp:284, please report a bug to PyTorch. (EncodeBlock at /pytorch/torch/csrc/jit/export.cpp:284)
frame #0: <unknown function> + 0x3fb4b7 (0x7ff7ddf9c4b7 in /home/schock/anaconda3/envs/pytorch_nightly/lib/python3.6/site-packages/torch/lib/libtorch.so.1)
frame #1: torch::jit::ExportGraph(std::shared_ptr<torch::jit::Graph> const&, std::vector<at::Tensor, std::allocator<at::Tensor> > const&, long, bool, torch::onnx::OperatorExportTypes) + 0x37 (0x7ff7ddf9c617 in /home/schock/anaconda3/envs/pytorch_nightly/lib/python3.6/site-packages/torch/lib/libtorch.so.1)
frame #2: <unknown function> + 0x374ee8 (0x7ff7debeeee8 in /home/schock/anaconda3/envs/pytorch_nightly/lib/python3.6/site-packages/torch/_C.cpython-36m-x86_64-linux-gnu.so)
frame #3: <unknown function> + 0xe12fd (0x7ff7de95b2fd in /home/schock/anaconda3/envs/pytorch_nightly/lib/python3.6/site-packages/torch/_C.cpython-36m-x86_64-linux-gnu.so)
frame #4: _PyCFunction_FastCallDict + 0x154 (0x55b6a21859e4 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #5: <unknown function> + 0x19cf4e (0x55b6a2212f4e in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #6: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #7: PyEval_EvalCodeEx + 0x966 (0x55b6a220e2f6 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #8: <unknown function> + 0x198bd6 (0x55b6a220ebd6 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #9: PyObject_Call + 0x3e (0x55b6a21857ee in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #10: _PyEval_EvalFrameDefault + 0x1abb (0x55b6a223910b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #11: <unknown function> + 0x196206 (0x55b6a220c206 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #12: <unknown function> + 0x1971cf (0x55b6a220d1cf in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #13: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #14: _PyEval_EvalFrameDefault + 0x10c5 (0x55b6a2238715 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #15: PyEval_EvalCodeEx + 0x329 (0x55b6a220dcb9 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #16: PyEval_EvalCode + 0x1c (0x55b6a220ea4c in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #17: <unknown function> + 0x1bf37b (0x55b6a223537b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #18: _PyCFunction_FastCallDict + 0x91 (0x55b6a2185921 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #19: <unknown function> + 0x19cdfc (0x55b6a2212dfc in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #20: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #21: <unknown function> + 0x196f8b (0x55b6a220cf8b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #22: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #23: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #24: <unknown function> + 0x196206 (0x55b6a220c206 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #25: <unknown function> + 0x1971cf (0x55b6a220d1cf in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #26: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #27: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #28: <unknown function> + 0x196f8b (0x55b6a220cf8b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #29: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #30: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #31: <unknown function> + 0x196f8b (0x55b6a220cf8b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #32: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #33: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #34: <unknown function> + 0x196206 (0x55b6a220c206 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #35: <unknown function> + 0x1971cf (0x55b6a220d1cf in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #36: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #37: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #38: <unknown function> + 0x196f8b (0x55b6a220cf8b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #39: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #40: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #41: <unknown function> + 0x196206 (0x55b6a220c206 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #42: <unknown function> + 0x1971cf (0x55b6a220d1cf in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #43: <unknown function> + 0x19ced5 (0x55b6a2212ed5 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #44: _PyEval_EvalFrameDefault + 0x2fa (0x55b6a223794a in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #45: PyEval_EvalCodeEx + 0x329 (0x55b6a220dcb9 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #46: PyEval_EvalCode + 0x1c (0x55b6a220ea4c in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #47: <unknown function> + 0x214c44 (0x55b6a228ac44 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #48: PyRun_FileExFlags + 0xa1 (0x55b6a228b041 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #49: PyRun_SimpleFileExFlags + 0x1c4 (0x55b6a228b244 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #50: Py_Main + 0x624 (0x55b6a228ed24 in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #51: main + 0xee (0x55b6a215675e in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)
frame #52: __libc_start_main + 0xe7 (0x7ff7f3404b97 in /lib/x86_64-linux-gnu/libc.so.6)
frame #53: <unknown function> + 0x1c847b (0x55b6a223e47b in /home/schock/anaconda3/envs/pytorch_nightly/bin/python)


Is this a known bug (and is there a workaround) or should I file an issue?