Torchscripting a module with _ConvNd in forward

Question

I am using PyTorch 1.4 and need to export a model with convolutions inside a loop in forward:

class MyCell(torch.nn.Module):
    def __init__(self):
        super(MyCell, self).__init__()

    def forward(self, x):
        for i in range(5):
            conv = torch.nn.Conv1d(1, 1, 2*i+3)
            x = torch.nn.Relu()(conv(x))
        return x


torch.jit.script(MyCell())

This gives the following error:

RuntimeError: 
Arguments for call are not valid.
The following variants are available:

  _single(float[1] x) -> (float[]):
  Expected a value of type 'List[float]' for argument 'x' but instead found type 'Tensor'.

  _single(int[1] x) -> (int[]):
  Expected a value of type 'List[int]' for argument 'x' but instead found type 'Tensor'.

The original call is:
  File "***/torch/nn/modules/conv.py", line 187
                 padding=0, dilation=1, groups=1,
                 bias=True, padding_mode='zeros'):
        kernel_size = _single(kernel_size)
                      ~~~~~~~ <--- HERE
        stride = _single(stride)
        padding = _single(padding)
'Conv1d.__init__' is being compiled since it was called from 'Conv1d'
  File "***", line ***
    def forward(self, x):
        for _ in range(5):
            conv = torch.nn.Conv1d(1, 1, 2*i+3)
                   ~~~~~~~~~~~~~~~ <--- HERE
            x = torch.nn.Relu()(conv(x))
        return x
'Conv1d' is being compiled since it was called from 'MyCell.forward'
  File "***", line ***
    def forward(self, x, h):
        for _ in range(5):
            conv = torch.nn.Conv1d(1, 1, 2*i+3)
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ <--- HERE
            x = torch.nn.Relu()(conv(x))
        return x

I have also tried pre-defining the conv's then putting them in a list inside __init__, but such a type is not allowed by TorchScript:

class MyCell(torch.nn.Module):
    def __init__(self):
        super(MyCell, self).__init__()
        self.conv = [torch.nn.Conv1d(1, 1, 2*i+3) for i in range(5)]

    def forward(self, x):
        for i in range(len(self.conv)):
            x = torch.nn.Relu()(self.conv[i](x))
        return x


torch.jit.script(MyCell())

This instead gives:

RuntimeError: 
Module 'MyCell' has no attribute 'conv' (This attribute exists on the Python module, but we failed to convert Python type: 'list' to a TorchScript type.):
  File "***", line ***
    def forward(self, x):
        for i in range(len(self.conv)):
                           ~~~~~~~~~ <--- HERE
            x = torch.nn.Relu()(self.conv[i](x))
        return x

So how to export this module? Background: I am exporting Mixed-scale Dense Networks (source) to TorchScript; while nn.Sequential may work for this simplified case, practically I need to convolve with all the historical convolution outputs in each iteration, which is more than chaining the layers.

Answer 1

You can use nn.ModuleList() in the following way.

Also, note that you can't subscript nn.ModuleList currently probably due to a bug as mentioned in issue#16123, but use the workaround as mentioned below.

class MyCell(nn.Module):
    def __init__(self):
        super(MyCell, self).__init__()
        self.conv = nn.ModuleList([torch.nn.Conv1d(1, 1, 2*i+3) for i in range(5)])
        self.relu = nn.ReLU()

    def forward(self, x):
        for mod in self.conv:
            x = self.relu(mod(x))
        return x

>>> torch.jit.script(MyCell())
RecursiveScriptModule(
  original_name=MyCell
  (conv): RecursiveScriptModule(
    original_name=ModuleList
    (0): RecursiveScriptModule(original_name=Conv1d)
    (1): RecursiveScriptModule(original_name=Conv1d)
    (2): RecursiveScriptModule(original_name=Conv1d)
    (3): RecursiveScriptModule(original_name=Conv1d)
    (4): RecursiveScriptModule(original_name=Conv1d)
  )
  (relu): RecursiveScriptModule(original_name=ReLU)
)