Fixed #913

[JinwooWang]

Dear Sir/Madam, This PR fixed the error: "local variable 'count_all' referenced before assignment" during evaluation mode, which has been claimed in #913 for a long time. I tested on my hardware configured with 3 GTX1080Ti: training an EfficientnetB5 with performing evaluation every 100 iterations using all three GPUs, and everything works fine. Thank you. CC @crcrpar @ptrblck

[minostauros]

Thanks a lot! CC @crcrpar @ptrblck

[Cucaracha18]

I have a question about track_runnung_stats == false

[jimmy19991222]

Thanks！