added *.stats.json file to checkpoint files

[brandondutra]

@nikhilk

example: with --checkpoints-to-keep 2 $ls -1 TOUT/checkpoints/ checkpoint model.ckpt-19068.data-00000-of-00001 model.ckpt-19068.index model.ckpt-19068.meta model.ckpt-19068.stats.json model.ckpt-20000.data-00000-of-00001 model.ckpt-20000.index model.ckpt-20000.meta model.ckpt-20000.stats.json

$ cat TOUT/checkpoints/*.stats.json { "eval_metric_value": 0.8999999761581421 }{ "eval_metric_value": 0.8999999761581421 }