Draft: Add Graphs as States for torchgfn

[alip67]

Description:
Unlike the current States object that necessitates appending dummy states to batch trajectories of varying lengths, our approach aims to support Trajectories through a nested Batch object representation. The Data class in Torch Geometric represents the graph structure, while the Batch class, which encapsulates batching of Data objects and their efficient indexing, represents the GraphStates object.

The current implementation of Trajectory supports the indexing dimensions: (Num time steps, Num trajectories, State Size). By using a nested Batch of Batch object to represent state Trajectories, the indexing would inherently take the form (Num trajectories, Num timesteps, State size). This approach requires implementing logic within _getitem_() and _setitem_() to internally.

To Do:
Compatibility check with Trajectories, Transition class

[josephdviviano]

note to self - we should evaluate if this can be vectorized.

[josephdviviano]

note to self - we should evaluate if this can be vectorized.

[saleml]

Thank you @younik and @alip67 for this important PR. Is there a script we can play with to see the training of the environment you created?

[younik]

Thank you @younik and @alip67 for this important PR. Is there a script we can play with to see the training of the environment you created?

There are still some issues to fix that prevent it from running properly.
I am working to fix them and will post a sample code for using it