Dgl error in pytorch-lightning

Hi, when running a code I wrote in dgl and pytorch-lightning this error happened. If I run the code in ddp on 1 GPU, then it works fine; when I run without ddp, it gives the following error. This shows that the error is on pytorch-lightning side, and how can I fix this? Thanks.

Validation sanity check: 0it [00:00, ?it/s]Traceback (most recent call last):
  File "main.py", line 50, in <module>
    trainer.fit(model, train_loader, val_loader)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 439, in fit
    results = self.accelerator_backend.train()
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/accelerators/gpu_accelerator.py", line 54, in train
    results = self.train_or_test()
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/accelerators/accelerator.py", line 66, in train_or_test
    results = self.trainer.train()
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 461, in train
    self.run_sanity_check(self.get_model())
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 647, in run_sanity_check
    _, eval_results = self.run_evaluation(test_mode=False, max_batches=self.num_sanity_val_batches)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/trainer/trainer.py", line 567, in run_evaluation
    output = self.evaluation_loop.evaluation_step(test_mode, batch, batch_idx, dataloader_idx)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/trainer/evaluation_loop.py", line 171, in evaluation_step
    output = self.trainer.accelerator_backend.validation_step(args)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/accelerators/gpu_accelerator.py", line 78, in validation_step
    output = self.__validation_step(args)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/accelerators/gpu_accelerator.py", line 84, in __validation_step
    batch = self.to_device(batch)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/accelerators/gpu_accelerator.py", line 113, in to_device
    return self.batch_to_device(batch, gpu_id)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/accelerators/accelerator.py", line 72, in batch_to_device
    return model.transfer_batch_to_device(batch, device)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/core/hooks.py", line 555, in transfer_batch_to_device
    return move_data_to_device(batch, device)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/utilities/apply_func.py", line 125, in move_data_to_device
    return apply_to_collection(batch, dtype=dtype, function=batch_to)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/utilities/apply_func.py", line 58, in apply_to_collection
    return elem_type([apply_to_collection(d, dtype, function, *args, **kwargs) for d in data])
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/utilities/apply_func.py", line 58, in <listcomp>
    return elem_type([apply_to_collection(d, dtype, function, *args, **kwargs) for d in data])
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/utilities/apply_func.py", line 49, in apply_to_collection
    return function(data, *args, **kwargs)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/pytorch_lightning/utilities/apply_func.py", line 122, in batch_to
    return data.to(device, **kwargs)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/dgl/graph.py", line 3858, in to
    self.ndata[k] = F.copy_to(self.ndata[k], ctx)
  File "/afs/ece.cmu.edu/usr/xujinl/anaconda3/envs/CSD/lib/python3.7/site-packages/dgl/backend/pytorch/tensor.py", line 90, in copy_to
    if ctx.type == 'cpu':
AttributeError: 'int' object has no attribute 'type'

Dear Chris_Liu,
Thanks you for using Pytorch Lightning !

It seems your data contains non-tensor element.

Please, join our Slack Channel for instant support !
Pytorch Lightning Team