TorchANI 2.6

This release constitutes an overhaul of the library, making it more modular and performant. There are a few backwards incompatible code changes, but most code will run with no changes.

Please consult the documentation for a comprehensive overview of the changes and new features, which include:

• Adding arbitrary analytic potentials to models
• Built in implementations of DFT-D3, GFN2-xTB repulsion, and other potentials
• Flexible model constructor for experimenting with different NN architectures
• Customizable AEV terms and cutoff functions
• Modular neighborlists for use with any model, including an accelerated $O(N)$ cell list and a Verlet list.
• Improved defaults of all models that make the PES smoother and better representative of the atomization limit
• torchani.datasets, to manipulate, create, and pre-batch ANI datasets
• Optimized CUAEV extension
• Optimized ensemble inference with BmmEnsemble
• Parallel calculations over elements using MultiNetParallel (MNP)

@IgnacioJPickering @yueyericardo, @khuddzu @nterrel