Model with stack does not work with int8 target type

[spacycoder]

Converting this dummy model with quantize_target_type="int8" and per_tensor=True throws an error in tflite

import torch.nn as nn
import torch
from tinynn.graph.quantization.quantizer import PostQuantizer
from tinynn.converter import TFLiteConverter

class StackModel(nn.Module):

    def forward(self, x: torch.Tensor):
        """
        Args:
            x: [N, H, W, C]
        """
        return torch.stack([-x, x], dim=-1)


def _main():
    dummy_input = torch.rand(1, 60,  60, 256).float()

    model = StackModel()

    qat_config = {
        "backend": "qnnpack",
        "per_tensor": True,
        "disable_requantization_for_cat": True
    }
    quantizer = PostQuantizer(
        model, (dummy_input,), work_dir="stack_model", config=qat_config
    )

    ptq_coarse_matcher = quantizer.quantize()
    ptq_coarse_matcher(dummy_input)

    with torch.no_grad():
        ptq_coarse_matcher.eval()
        ptq_coarse_matcher.cpu()

        ptq_coarse_matcher = quantizer.convert(ptq_coarse_matcher)
        torch.backends.quantized.engine = quantizer.backend
        converter = TFLiteConverter(
            ptq_coarse_matcher,
            (dummy_input),
            "stack_model.tflite",
            fuse_quant_dequant=True,
            quantize_target_type="int8"
        )
        converter.convert()

if __name__ == '__main__':
    _main()

Tflite error:

    return self._interpreter.AllocateTensors()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: /tensorflow/tensorflow/lite/kernels/concatenation.cc:184 t->params.zero_point != output->params.zero_point (-1 != 0)Node number 3 (CONCATENATION) failed to prepare.

Note that the model works fine if I remove the "negative x" and instead send the same tensor twice, and it works with uint8