the bug of the CL4SREC code

[ShelveyJ]

In the augmentation phase of CL4SREC, which includes three tasks ,i.e. item_crop、item_mask、item_reorder, the code of item_mask and item_reorder unexpectedly changes the source seq, resulting in the augmented sequence being identical to the original sequence.
The main reason is that the slicing operation is performed on the tensor instead of the list.