Update tpu_estimator.py

Fixing #43. This does not affect the current repo because it is not used.

tpu_estimator.py

@@ -2899,7 +2899,7 @@ def multi_tpu_eval_steps_on_single_shard():
       outputs_from_all_shards=False,
       device_assignment=ctx.device_assignment)
 
-  loss = rte[0]
+  loss = ret[0]
   scaffold = _get_scaffold(captured_scaffold_fn)
   return compile_op, loss, host_calls, scaffold, captured_eval_hooks.get()