[ET-VK][Ez] Improve vec class (#4635)

Context
This diff adds some additional API functions to the utils::vecN family of classes. The following improvements were made:

Added overloaded assignment operator, allowing for vec_instance = vec_instance_2
Added overloaded indexing operator, allowing for vec_instance[2] instead of having to do vec_instance.data[2]
Note that the large number of changes are due to replacing .data[ with [ throughout the codebase.



Differential Revision: D60931001

Pull Request resolved: https://github.com/pytorch/executorch/pull/4592

Co-authored-by: Stephen Jia <ssjia@meta.com>