[Tests] Update operator tests forwardDims/forward for scalars

Activate previously commented tests on scalars for
elementwise operators: Test_[Div|Mul|Sub|Pow]_Op.cpp.
Add scalar test for Test_MatMul_Op.cpp.
Add negative scalar tests for Test_[Concat|Transpose]Impl.cpp.
Update implementations forward/forwardDims of operators
Concat|Transpose|MatMul to assert on unexpected scalar inputs.