PLease make the code to support the situation that action classes are less than 5

If my action class num is less than 5, the top5 make no sense, and it will report error : "RuntimeError: invalid argument 5: k not in range for dimension at /pytorch/aten/src/THC/generic/THCTensorTopK.c". If i change (1,5) to (1) at [here](https://github.com/facebookresearch/SlowFast/blob/fd41618191d3c21c1ca21a61369ce9917646cf9c/tools/train_net.py#L113), it will cause continuous errors, so the code need to be more flexbile.