Fix %hhd format specifiers as PRId8 (#470)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/470

`PRId8` should be used with `int8_t` instead of `%hhd`. Additionally, fix `%hdd` to `%hhd`. `%hdd` is an invalid format and was being interpreted as `short` when it should also be `int8_t`.

Reviewed By: JacobSzwejbka

Differential Revision: D49568420

fbshipit-source-id: dba672af4501d954195f00f6687365b374d7db4d