Standardize Atacformer and ScEmbed API

[nleroy917]

Atacformer and scEmbed should probably have similar APIs. More specifically, the scEmbed model should probably be refactored to use huggingface infra and then use the EncodeTokenizedCells mixin I made for Atacformer

[nleroy917]

I think this will actually be pretty easy. There are two steps to this I think.

First, take the current scEmbed class and change it to inherent from PreTrainedModel from huggingface.
Second, change the train function to transfer weights to this new class and call save_pretrained instead. Then this can be pushed to huggingface and used for model instantiation later on