Add on-device sampling support for Qwen3VL Dense

[quic-xiyushi]

This PR enables on-device sampling for Qwen3‑VL Dense.

• The on-device sampling infrastructure is generic, but new models must be registered in SamplerTransform.
• Qwen3‑VL Dense introduces deepstack features that were not part of previously supported model inputs/outputs.
• This PR updates the sampler transform to correctly handle those deepstack features.

These changes make Qwen3‑VL Dense compatible with on-device sampling.

[vbaddi]

@quic-xiyushi Can we pls add a test case for this, thanks

[quic-xiyushi]

@vbaddi , done, I have added a test case for qwen3vl dense with on-device sampling. Thanks!

[quic-xiyushi]

@vbaddi , this change should go into both the main branch and the release/v1.21.6 branch. Do I need to create two PRs for that?

[vbaddi]

@vbaddi , this change should go into both the main branch and the release/v1.21.6 branch. Do I need to create two PRs for that?

yes, we would need it on both release and main branch too.
++ @quic-rishinr

[quic-hemagnih]

@quic-xiyushi please raise a PR for main branch also