[Proposal] Add MLP transcoders

[dtch1997]

Proposal

Support training, loading, and inference of MLP transcoders.

Motivation

MLP transcoders were trained by Jacob Dunefsky and Philippe Chlenski and have been shown to be useful.

Pitch

• Implement a HookedTranscoder class analogous to HookedSAE and using similar functionality.
• Implement a transcoder training runner.
• Support loading pre-trained transcoder checkpoints.

Checklist

• [X] I have checked that there is no similar issue in the repo (required)

[dtch1997]

I'm likely going to be rather hacky with the first pass of the implementation, and possibly duplicate a bunch of code in order to have maximum freedom to make changes. Refactoring can be done later.