[BE][optim] Remove objects from being globals and comment to clarify (#100899)
What happened in this PR?
1. Added comments to clarify rosenbrock
2. Moved global objects to be within classes for better readability/grouping
3. Renamed dnn to cnn for consistency
This is the very first of the cleanup of test_optim.py
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100899
Approved by: https://github.com/albanD, https://github.com/Skylion007
