ONNX: Invalid int64 value

[jerryhatestom]

I have added a self-defined attribute to ONNX OP Cast with type int64[], it is expected to show as -1,32, but it shows as 18446744073709551615,32(with underline in below picture). I guess -1 is represented as unsigned int64, since 18446744073709551615 is the max unsigned int64 number.

The issue can be found in version v7.6.4, and can not be found in old version v7.3.5.