data_time, losses etc are of type
class AverageMeter(object):
    """Computes and stores the average and current value"""
    def __init__(self):
        self.reset()

    def reset(self):
        self.val = 0
        self.avg = 0
        self.sum = 0
        self.count = 0

    def update(self, val, n=1):
        self.val = val
        self.sum += val * n
        self.count += n
        self.avg = self.sum / self.count

It’s thus just used to compute and store some statistics, e.g. about the loss.
It’s being updated e.g. in these lines: https://github.com/pytorch/examples/blob/master/imagenet/main.py#L199 675