Adding tensorflow summary metrics support for Tensorboard

[dpsm]

Adding output folder option for storing training artifacts

Fixing plotting code issue when restarting training from
an offter > 0.

[starkcn]

hello，have you solved this problem?i have the same problem.

[yahuuu]

@starkcn I'm nobody, but I meet the problems of @dpsm, U could solve like this:
ax3.plot(range(args.epoch_start_i, epoch+1, 1), avg_iou_per_epoch)