Add VLM export support (SmolVLM as starting point)

Hey, I recently opened a PR in huggingface/transformers (#46360) adding vision/audio input support to the ExecuTorch export pipeline in executorch.py. The reviewer (zucchini-nlp) closed it and pointed me here, mentioning that multimodal ExecuTorch support is moving to optimum-executorch.

Looking at the repo, I can see text LLMs and image classifiers are both supported, but there's no export path for vision-language models, models that take image + text and generate text. SmolVLM seems like an obvious first target: it's a HF model, it's small enough to actually run on-device.

Before I start on anything, a few things I'd want to confirm:

Is VLM export support already being worked on somewhere, or is it an open gap?
Would the right shape be a new ExecuTorchModelForImageTextToText class following the ExecuTorchModelForCausalLM pattern?
Any known blockers on the ExecuTorch side for exporting a vision encoder + LLM decoder in one graph?

Happy to prototype something if this direction makes sense, just don't want to duplicate work.