IntelGPU backend: Power, Sigmoid and ReluBackprop operations (#1286)
IntelGPU backend: Power, Sigmoid and ReluBackprop operations (#1286)

* IntelGPU backend: Power, Sigmoid and ReluBackprop operations

* style changed to ReluBackprop

* Update intelgpu_backend.cpp

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