Remove contrast_target_prefixes from contrastive step functions

[gsarti]

Description

This PR removes the contrast_target_prefixes argument from all contrastive functions and implements default right-side alignment of contrastive sequences. This allows a more intuitive usage of contrastive step functions that 1) doesn't require splitting manually the contrastive target prefix and 2) works out of the box for decoder-only LMs:

Example before:

mt_model.attribute(
    "she started working as a cook in London.",
    "ha iniziato a lavorare come cuoca a Londra.",
    contrast_sources="After finishing her studies, she started working as a cook in London.",
    contrast_target_prefixes="Dopo aver terminato gli studi, ",
)

After:

mt_model.attribute(
    "she started working as a cook in London.",
    "ha iniziato a lavorare come cuoca a Londra.",
    contrast_sources="After finishing her studies, she started working as a cook in London.",
	contrast_targets="Dopo aver terminato gli studi, ha iniziato a lavorare come cuoca a Londra.",
)