replace vec::vec_scalar_t with at::opmath_type (#91086)
### Motivation
The two accumulation types vec::vec_scalar_t and at::opmath_type are duplicated, so we replace vec::vec_scalar_t with at::opmath_type, and vec::vec_scalar_t will be deprecated later.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91086
Approved by: https://github.com/jgong5, https://github.com/mingfeima
