refactor validation perplexity test; check perplexity every 500000 minibatches.
refactor validation perplexity test; check perplexity every 500000 minibatches.

graehl/nplm,rsennrich/nplm,graehl/nplm,rsennrich/nplm,graehl/nplm,rsennrich/nplm,graehl/nplm,graehl/nplm,rsennrich/nplm,rsennrich/nplm