Move `ATen/core/DimVector.h` to `c10/util/DimVector.h`.
This PR moves `ATen/core/DimVector.h`, as suggested in:
https://github.com/pytorch/pytorch/pull/76812#discussion_r866875924
The changes can be summarized as:
- Changing includes from `ATen/core/DimVector.h` to `c10/util/DimVector.h`
- Re-declaring both the type and constant size in `at` namespace
- Making `c10::contiguous_strides` return a `DimVector`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77045
Approved by: https://github.com/peterbell10, https://github.com/ezyang
