[loss] Allow no loss in the model

[loss] Allow no loss in the model

Allow setting no loss in the model
This allows inferencing a model, and creating submodels
with backbones to infer some particular output features

Updated existing unittest with no loss to succeed
and added more unittests to validate that models run successfully
without loss

V2:
Updated mnist_inf.ini to be without loss

**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>