[async-rl CL 8/n] : Wait to collect trajectories for rl_trainer.
[async-rl CL 8/n] : Wait to collect trajectories for rl_trainer.

This comes in handy when the rl_trainer itself restarts due to rescheduling
and its wait time is exhausted.

A `wait_forever` is appropriate here, since otherwise no progress can be made.

PiperOrigin-RevId: 269091806

tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor,tensorflow/tensor2tensor