Add an option to choose the I/O Device for saving and loading models for CheckpointManager.
Add an option to choose the I/O Device for saving and loading models for CheckpointManager.

This option enables saving and restoring models to or from filesystems only
accessible from the localhost when using multiple devices.

PiperOrigin-RevId: 346246041
Change-Id: Ic30e7e178f66b43f15c0b04c989d2f9690d42449

tensorflow/agents,tensorflow/agents