[Layer] Update forward and backward layer functions

[Layer] Update forward and backward layer functions

Update the forwarding and backwarding layer functions to support
multiple inputs and outputs.
This is crucial to support concat, element wise add, etc kind of layers
as well as skip connection.
Further, forward no longer returns status but rather throws.

**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>