Update l2_norm and std_dev builders to use op::Sqrt (#2040)
Update l2_norm and std_dev builders to use op::Sqrt (#2040)

* Update l2_norm and std_dev builders to use op::Sqrt instead of op::Power(...,0.5)

* Removed unneeded power.hpp include

NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph