PCA center, GGUF import

[vgel]

Changes

• Adds a new method argument to ControlVector.train. This defaults to the old method, method="pca_diff", but it can be switched to method="pca_center" for more stability. This seems to be necessary for training vectors on larger models, such as llama3-70b. Thanks to @r3ndd
• Adds ControlVector.import_gguf as a counterpart to ControlVector.export_gguf. GGUF is now the preferred way to save control vectors, for better compatibility with llama.cpp (however, the old pickle method will continue to work)
• Adds eq implementation for ControlVector
• More thorough testing

Co-Authored-With: r3ndd@users.noreply.github.com

[lin72h]

very impressive, thanks for working on this feature