Refactor c10::complex and cleanup c10::Scalar (#38593)
Summary:
**Main:**
- `c10::complex` is refactored: it no longer uses inheritance to specialize constructors, but using SFINAE instead. This implementation is cleaner and avoids some compiler bugs.
- `c10::Scalar` is cleaned up: it no longer needs to store complex as `double z[2]`, `c10::complex<double>` will work.
**Other cleanups:**
- `numeric_limits` of `c10::complex` is moved to `complex_utils.h`
- the variable in `c10::complex` storing real and imag is changed from `storage[2]` to `real_` and `imag_`
- remove the `c10::` before `complex` when in `c10` namespace
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38593
Differential Revision: D21769463
Pulled By: anjali411
fbshipit-source-id: 3cb5bcbb0ff304d137221e00fe481a08dba7bc12
