Use `int8_t` instead of `char` in [load|store]_scalar` (#52616)
Summary:
Since `char` is not guaranteed to be signed on all platforms (it is unsigned on ARM)
Fixes https://github.com/pytorch/pytorch/issues/52146
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52616
Test Plan: Run ` python3 -c "import torch;a=torch.tensor([-1], dtype=torch.int8);print(a.tolist())"` on arm-linux system
Reviewed By: walterddr
Differential Revision: D26586678
Pulled By: malfet
fbshipit-source-id: 91972189b54f86add516ffb96d579acb0bc13311
