Added pos Node Type for Diffusion Modeling Support in HydraGNN

[zachfox]

Description:

This PR incorporates Bareesh and Eric's updates to add a pos node type. This helps us to implement diffusion models on top of HydraGNN.

Key changes include:

• Defined and integrated pos as a new node type for diffusion processes (mostly done by Eric Yeats and Bareesh Bhaduri, but git histories could not easily be reconciled).
• Updated data handling, configuration, and model processing to accommodate pos.
• Ensured compatibility with HydraGNN's
• Added models/HybridEGCLStack.py - allows for some heads have a cross-entropy loss and some to have mse loss.