Display individual losses when forwarding net

When there are multiple loss layers, the solver will display the total loss as well as the individual losses to allow easier inspection.

Very useful if there are multiple loss layers because there can be a lot of problems. The different loss layers may reduce at vastly different speed, or certain type of loss dominates the total such that other layers are basically ignored, etc.

Sample output

```
I1016 14:38:03.321640 21826 solver.cpp:275] Iteration 0, Testing net (#0)
I1016 14:38:11.277034 21826 solver.cpp:326]     Test net output #0: accuracy = 0.00195312
I1016 14:38:11.370230 21826 solver.cpp:213] Iteration 0, loss = 9.92332
I1016 14:38:11.370256 21826 solver.cpp:217]     Layer "loss": 5.29832
I1016 14:38:11.370275 21826 solver.cpp:217]     Layer "intermediate_loss": 4.62499
I1016 14:38:11.370292 21826 solver.cpp:456] Iteration 0, lr = 0.03
I1016 14:38:16.585816 21826 solver.cpp:213] Iteration 20, loss = 8.80078
I1016 14:38:16.585873 21826 solver.cpp:217]     Layer "loss": 5.28885
I1016 14:38:16.585891 21826 solver.cpp:217]     Layer "intermediate_loss": 3.51193
I1016 14:38:16.585901 21826 solver.cpp:456] Iteration 20, lr = 0.03
I1016 14:38:21.799321 21826 solver.cpp:213] Iteration 40, loss = 8.25201
I1016 14:38:21.799370 21826 solver.cpp:217]     Layer "loss": 5.26909
I1016 14:38:21.799386 21826 solver.cpp:217]     Layer "intermediate_loss": 2.98292
```