[optimizer] Simplify optimizer initialize

[optimizer] Simplify optimizer initialize

As there is just one optimizer and shared by layers, it must be initialized just once by the neural network.
Also, addOptimizerVariables() moved out separately from initialize() as initialize() should work
on optimizers parameters and should not need list of weights.

Also remove set_tensor argument which was redundant

**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>