fix circular import for layer_cpu.py (#907)
fix circular import for layer_cpu.py (#907)

* removed explicit import of the Layer class in layer_cpu.py to avoid circular import to resolve issue #906
* added the is_mklop attributes to layer_cpu.py

NervanaSystems/neon,NervanaSystems/neon,NervanaSystems/neon,NervanaSystems/neon