[Loss] Apply requireLabel to feed label

[Loss] Apply requireLabel to feed label

Currently, loss layer is the only one that can accept label which was
prohibiting building a custom loss layer.

**Major Changes**
- Apply requireLabel to feed label
- Last layer is not assumed loss layer when feeding label(still it is in
other places, need to be dealt)

**Minor Changes**
- Feeding multiple label is now possible (spliting label is future work)
- [Fix] Count Trainable includes loss layer
- [Fix] Handle cases When all layer is not trainable for
neuralnet::backwarding

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>