Resolve "[Add][Feature][Learning] Learning Rate Scheduler"

Closes #80 (closed)

Tasks

• Flexible creation of update function
• Common LRScheduler class
  • ConstantLR
  • StepLR
• implement LR
  • LRScheduler::update() for each learning step
  • LRScheduler::lr_profiling() to predict lr evolution through epochs
• implement warmup
• unit-tests