I had a similar problem with my model. Try to experiment with the input tensor you use in your Swift code. In my case I replaced the input tensor with a random one and then it worked.
at::Tensor tensor = torch::rand({ 1, 3, input_height, input_width }, at::kFloat).metal();

I don’t know how to go on from there, but maybe this leads you on the right track. Let me know if you find out more.