[network] Rework the backwarding

[network] Rework the backwarding

- remove forwarding from backwarding
backwarding should just do backwarding and no more
- moved backwarding back to neuralnetwork so that graph
does not has to care about how to backward etc.
Graph just provides iterators for iterating the graph
in reverse. Graph does not know that layers have backwarding etc.

Also this removes dependency of graph from optimizer.

V2:
Added comment fixes for the corresponding PR

**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>