[optimizer] Refactor optimizer

[optimizer] Refactor optimizer

This patch refactors optimizer in the following fashion:
- split optimizer implementations of different types to derived classes of adam and sgd
- create a optimizer_factory to create the optimizer class objs based on its type
  - this can be directly used with ccapi
- OptParam struct has been removed
- applyGradients has been broken down into different methods
- updated associated unittests

**Self evaluation:**
1. Build test: [x]Passed [ ]Failed [ ]Skipped
2. Run test: [x]Passed [ ]Failed [ ]Skipped

Signed-off-by: Parichay Kapoor <pk.kapoor@samsung.com>