[MPS] Workaround for uint8 gather bug (#82315)
Looks like MPSGraph generates a wrong shader for expanding column into a matrix, though datatype should not really matter for scatter-gather ops, should it?
Also, fix minor typo: `c10::ScalarType:Char` is signed type, whereas `c10::ScalarType::Byte` is unsigned.
Workarounds https://github.com/pytorch/pytorch/issues/82305 by aliasing tensors of `uint8` types to `int8` typed ones
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82315
Approved by: https://github.com/janeyx99, https://github.com/kulinseth
