Write a utility for merging sampled rollouts into a single file

[jbloomAus]

We can sample rollouts using sample from agents. However, it would be good to be able to merge trajectory datasets. In order to do this, it might be worth cleaning up the offline_dataset code and working out the right way to do this. A fast way would be to use torch utils to concat datasets but this break the visualizer we have for the dataset. Possible not worth doing anything else.

https://github.com/jbloomAus/DecisionTransformerInterpretability/blob/4c652db69ab55aa5e1cf8695b53102b15fe9608b/src/ppo/agent.py#L829-L857