[ coverity ] Fix Coverity issue
Fix Coverity issue on
- /test/unittest/layers/layers_golden_tests.cpp
- /test/unittest/models/unittest_models_recurrent.cpp
- /test/unittest/unittest_nntrainer_models.cpp
Resolves:
```
Use of auto that causes a copy (AUTO_CAUSES_COPY)
auto_causes_copy: This lambda has an unspecified return type
copy: This return statement creates a copy.
```
**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped
Signed-off-by: Donghak PARK <donghak.park@samsung.com>