Change osbervable names `itemsession_<obs_name>` to `sessionitem_<obs_name>`.

[TianyuDu]

The ChoiceDataset is expecting a tensor of shape (S, I, *) for observables depending on both item and session. The shape of tensors is session-item-*, but right now such a variable is called itemsession_obs. We need to change this to sessionitem_obs to make it more intuitive.

[TianyuDu]

Ideally, we support both itemsession_obs and sessionitem_obs for backward compatibility.

[TianyuDu]

Ideally, both itemsession_obs and sessionitem_obs should work. Similarly, both useritem_obs and itemuser_obs should work.

1. Both itemsession_obs and sessionitem_obs work exactly the same.
2. Both useritem_obs and itemuser_obs should work exactly the same.
3. All strings starting with any of the 6 permutations of user, item, and session should be parsed exactly the same.