[MPS] Fix torch.uint8 support (#80049)
`ScalarType.Byte` should be cast to `MPSDataTypeUInt8`
And support for `torch.int8` as well as test those conversions in `TestMPS.test_to`
Fixes #80006
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80049
Approved by: https://github.com/albanD
