How to merge p_crops into a batch?

[Korverf]

Thanks for your great work! Since you have not released the training code, I write a training code based on FairMOT. However, I meet a problem when using the dataloader:
RuntimeError: invalid argument 0: Sizes of tensors must match except in dimenson0. Got 4 and 5 in dimenson 1 at /opt/conda/conda-bld/pytorch_1579022034529/work/aten/src/TH/generic/THTensor.cpp:612

And I find the reason is that the size of p_crops is various in different pics(it depends on the number of objects in the picture)，so I cannot use dataloader to stack the p_crops into a batch. Can you tell me how to merge p_crops into a batch?