[Weights] Split weight variable init and alloc

[Weights] Split weight variable init and alloc

Split the initialization and memory allocation for weights
3 exposed bugs with this has been resolved:
- manager does not allow tracking var_grads once initialized
- resetGradient confirms the allocation is done before accessing the memory
- allocateVariable() calls the correct initializer now

Further, the logic of reinitialize in unittest for layers has been
split into two parts - initialize and reinitialize where
reinitialize will reset layer and manager and then call initialize.

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