[unstable branch] validation frequency adding to config.json

[yangtuo250]

Is your feature request related to a problem? Please describe.
Validation per epoch will slow down training process severely when validation dataset is large.

Describe the solution you'd like
Validation frequency can be modified in config.json

[AIWintermuteAI]

I merged your PR. The feature might be useful, I'll see if I propagate it to the master branch in the future.