Fix memory leak in LTRTrainer._update_stats().

[Capricorn231]

The method _update_stats() is called with val, which is a Tensor with gradients. When use with AverageMeter, the gradient graph will be accumulated during training and won't be released and collected by GC, causing significant memory leak.

[martin-danelljan]

val is not necessarily always a tensor. So we need to have a check if its actually a tensor first.