[Fix] Fix device mismatch for ascend npu when loss_cfg.loss_reduction != "token"

[nil0x9]

Currently when one passes loss_cfg.loss_reduction other than "token" on a ascend/npu device, a Runtime Error (device mismatch) is expected in this line:

xtuner/xtuner/v1/loss/ce_loss.py

Line 191 in d339718

loss = (loss * loss_weight).sum()

The root cause of this error is that, in ascend npu device, cu_seq_lens tensors are required to be on cpu. In func build_batches_loss_kwargs, the devuce ofloss_weight is inherited from num_grad_tokens -> boundaries -> cu_seq_lens -- and hence the problem.