I found that when I del the tensor, the GPU still has a small amount of memory occupation.
The codes are below:

import torch
t = torch.zeros([1024, 1024, 1024, 2], device=‘cuda:0’)
del t
torch.cuda.empty_cache()

The GPU still has about 700M usage
|==================+=============+==================|
|   0  GeForce RTX 208…  Off  | 00000000:05:00.0 Off |                  N/A |
| 33%   49C    P8    29W / 250W |    717MiB / 11016MiB |      0%      Default |
±------------------------------±---------------------±--------------------------------+