Two fixes, some minor improvements

[chgaul]

Dear authors,

I made the following changes:

• Fixed models MGT and CustomMGT for pe_name = "learnable_equiv_wave" (self.lin_pos is needed, but not instantiated). Please check it is correct like this! (The other solution would have been to modify the forward function.)
• Fixed pre_transform in the polymer dataset.
• Speed-up of the wavelet pre_transform: The scipy.linalg.expm operation is much faster for dense matrices than for sparse matrices (approx factor 10 on my hardware).
• Some minor improvements in style and readability. Please have a look, and, if it makes sense to you, merge it into your main branch.

Regards, Christopher

[nnkhang19]

Thanks a lot @chgaul! We appreciate your intensive efforts in helping improve our released source code. After comparing your commits with the original code, we decided to merge yours into our main repository.