Change data representation in collate_fn to use lists of integers

[mauriciogtec]

There is no need to use padding for cell membership in the loader representations. Cell membership padding is not used for spatial tasks, which also have huge differences in cell sizes. The data collider supports lists of integers. Padding can be created efficiently on the fly with torch functions for qm9.

This issue should:

1. Represent internally as a list of integers in the collate fn
2. For qm9, restore the original padded format for backward compatibility

After this issue is solved, other issues/PRs will integrate the spatial benchmarks.

[mauriciogtec]

Maybe the solution is to have two collators

[mauriciogtec]

Fixed in d736879.