patch for pyg dataloader pynb; remove 404 link

[FeynmanDNA]

removed the 404 links for synthetic dataset: https://syncandshare.lrz.de/dl/fiYbEo8Vv1mpHWtT2ShRqB3i/synthetic.zip in pytorch_dataset folder code

patched the toy example notebook which is actually mentioned in README (see screenshot):

• separate the ogb dataloader code from the pyg dataloader code
• add source. prefix to the custom code dirs in source

[FeynmanDNA]

@suprosanna please help to review the changes to your notebook, thanks