When a mirrored variable is fetched in cross-tower mode, fetch its primary variable.

When a mirrored variable is fetched in cross-tower mode, fetch its primary variable.

This prevents errors like
ValueError: Fetch argument MirroredVariable({'/job:localhost/replica:0/task:0/device:GPU:0': <tf.Variable 'global_step:0' shape=() dtype=int64>, '/job:localhost/replica:0/task:0/device:GPU:1': <tf.Variable 'global_step/replica_1:0' shape=() dtype=int64>}) cannot be interpreted as a Tensor. (Device /job:localhost/replica:0/task:0/device:CPU:0 not found in ['/job:localhost/replica:0/task:0/device:GPU:0', '/job:localhost/replica:0/task:0/device:GPU:1'] (current device ))

I ran distribute/examples/resnet with and without the change and it fixed the problem.

PiperOrigin-RevId: 194828672