[DCP][Test] Fix device assignment in test/distributed/checkpoint/test_file_system_checkpoint_cpu.py (#109141)
Device should always be "cpu" for cpu tensor types.
This will fix fb buck test failure when running in internal.
```
buck2 test '@fbcode//mode/dev-nosan' fbcode//caffe2/test/distributed/checkpoint:file_system_checkpoint_cpu -- --exact 'caffe2/test/distributed/checkpoint:file_system_checkpoint_cpu - test_switch_between_sharded_tensor_to_tensor_thread_count_1 (test_file_system_checkpoint_cpu.TestDistributedReshardOnLoad)'
```
This will unblock [D48667323](https://www.internalfb.com/diff/D48667323).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/109141
Approved by: https://github.com/fegin
