Revert cupy.cuda.Device behavior to v9 - #7427

# Note: requires 3 GPUs

import cupy
import torch


dev1 = cupy.cuda.Device(1)

with dev1:
    t1 = torch.arange(10, device='cuda')
    assert 'cuda:1' == str(t1.device)
    assert ((t1 - torch.from_dlpack(cupy.arange(10))).sum() == 0).item() is True
    with torch.cuda.device(2):
        t2 = torch.arange(10).cuda()
        assert 'cuda:2' == str(t2.device)
        assert cupy.cuda.Device().id == 2
        with dev1:
            assert 1 == torch.cuda.current_device()
    assert cupy.cuda.Device().id == 1
assert 0 == torch.cuda.current_device()