integrate mock vision backbone into model

[epwalsh]

Adds an abstraction for vision backbones, along with a mock dataset to test it. You can run a test training job on 2 GPUs with the mock dataset like so:

torchrun --nproc-per-node=2 scripts/train.py configs/mm-tiny.yaml

[notion-workspace[bot]]

Integrate mock vision backbone into OLMo

[notion-workspace[bot]]

Figure out how to put the image data into the sequences