ONNX convert BERT-pytorch

I’ve posted this question on SO (https://stackoverflow.com/questions/56962116/bert-pytorch-to-onnx-conversion-lambda-error), but I thought this might be a better place for it. I’m trying to convert the PyTorch BERT implementation from https://github.com/codertimo/BERT-pytorch, but I’m hitting an error with the TransformerBlock:

builtins.ValueError: Auto nesting doesn't know how to process an input object of type bert_pytorch.model.transformer.TransformerBlock.forward.<locals>.<lambda>. Accepted types: Tensors, or lists/tuples of them

The code for the TransformerBlock is:

class TransformerBlock(nn.Module):
    Bidirectional Encoder = Transformer (self-attention)
    Transformer = MultiHead_Attention + Feed_Forward with sublayer connection

    def __init__(self, hidden, attn_heads, feed_forward_hidden, dropout):
        :param hidden: hidden size of transformer
        :param attn_heads: head sizes of multi-head attention
        :param feed_forward_hidden: feed_forward_hidden, usually 4*hidden_size
        :param dropout: dropout rate

        self.attention = MultiHeadedAttention(h=attn_heads, d_model=hidden)
        self.feed_forward = PositionwiseFeedForward(d_model=hidden, d_ff=feed_forward_hidden, dropout=dropout)
        self.input_sublayer = SublayerConnection(size=hidden, dropout=dropout)
        self.output_sublayer = SublayerConnection(size=hidden, dropout=dropout)
        self.dropout = nn.Dropout(p=dropout)

    def forward(self, x, mask):
        x = self.input_sublayer(x, lambda _x: self.attention.forward(_x, _x, _x, mask=mask)) // <-- Error!
        x = self.output_sublayer(x, self.feed_forward)
        return self.dropout(x)

Any workarounds?