Describes some metadata about the TPU system.

num_cores interger. Total number of TPU cores in the TPU system.
num_hosts interger. Total number of hosts (TPU workers) in the TPU system.
num_of_cores_per_host interger. Number of TPU cores per host (TPU worker).
topology an instance of tf.tpu.experimental.Topology, which describes the physical topology of TPU system.
devices a tuple of strings, which describes all the TPU devices in the system.