[weight/var_grad] Remove exposure of weight/var_grad

[weight/var_grad] Remove exposure of weight/var_grad

This patch updates the usage of weight and var_grad headers in order to
hide them internally and not expose them in the devel headers.
The below changes are made to support this:
- weight and var_grad specs are declared separately in tensor wrapper
specs header.
- layer_context is made indepedent of the weight and var_grad
definition. The usages and implementation are moved to layer_context
souce file.

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