add step3v in VLMS
Author: yhyang201
Pull request: yes
Labels: name: New model

# What does this PR do?

I would like to kindly request the addition of a new VLM entry, `"step3v"`, at the following location:
[https://github.com/huggingface/transformers/blob/2c0af41ce5c448f872f3222a75f56030fb2e5a88/src/transformers/modeling\_utils.py#L233](https://github.com/huggingface/transformers/blob/2c0af41ce5c448f872f3222a75f56030fb2e5a88/src/transformers/modeling_utils.py#L233)

The reason for this request is that `step3v` requires the use of `_checkpoint_conversion_mapping` during model loading. However, as per the current implementation at:
[https://github.com/huggingface/transformers/blob/2c0af41ce5c448f872f3222a75f56030fb2e5a88/src/transformers/modeling\_utils.py#L4578](https://github.com/huggingface/transformers/blob/2c0af41ce5c448f872f3222a75f56030fb2e5a88/src/transformers/modeling_utils.py#L4578)
only models registered in `VLMS` are allowed to access `_checkpoint_conversion_mapping`.

By including `step3v` in the `VLMS` list, it will be possible to remove the current workaround in the [[example code](https://huggingface.co/stepfun-ai/step3/blob/main/README.md?code=true#L344)](https://huggingface.co/stepfun-ai/step3/blob/main/README.md?code=true#L344), where key mappings are manually passed in. This change would help improve the readability and overall clarity of the example.

Thank you very much for your time and consideration!

@amyeroberts, @qubvel

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Comments:
hey @yhyang201! Btw, we now have a public weight mapping in https://github.com/huggingface/transformers/blob/main/src/transformers/conversion_mapping.py and are planning to allow users to auto-convert weights on the fly for any model. Might be of interest to you :)