Add a `flush` after each `create_item` to the example in `TrajectoryWriter`.
Add a `flush` after each `create_item` to the example in `TrajectoryWriter`.

https://github.com/deepmind/reverb/issues/92

PiperOrigin-RevId: 429986171
Change-Id: I9cb2749d2bdffa0d5ad8fb44195173be7290fe43

deepmind/reverb,deepmind/reverb,deepmind/reverb,deepmind/reverb