[C++ API Parity] [Optimizers] Merged Optimizer and LossClosureOptimizer (#34957)
Summary:
1. Removed LossClosureOptimizer, and merged Optimizer into OptimizerBase (and renamed the merged class to Optimizer)
2. Merged the LBFGS-specific serialize test function and the generic test_serialize_optimizer function.
3. BC-compatibility serialization test for LBFGS
4. Removed mentions of parameters_ in optimizer.cpp, de-virtualize all functions
5. Made defaults_ optional argument in all optimizers except SGD
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34957
Test Plan: Imported from GitHub, without a `Test Plan:` line.
Differential Revision: D20518647
Pulled By: anjali411
fbshipit-source-id: 4760d1d29df1784e2d01e2a476d2a08e9df4ea1c
