[skyrl][tinker] Multi-modal Tinker Sampling

[nithinvc]

Summary

Adds VLM sampling support to RemoteInferenceClient sample endpoint. Finalizes inference side changes for #1200.

• Extract _render_for_sample to handle both text-only and image-containing prompts. For text-only prompts, it flattens chunk tokens directly. When images are present, it calls /v1/chat/completions/render to process images, then splices placeholder tokens into the pre-tokenized text stream with adjusted offsets.
• Update sample() to pass multi-modal features through to the generate payload when present.

Test plan

• Verify text-only sampling still works (no render call made, features is None)
• Verify image sampling works end-to-end (render call is made, placeholder tokens are spliced correctly, features are included in the generate payload)
• New multi-modal sampling tests in test_vlm_inference_generation.py

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 9 additional findings in Devin Review.

[devin-ai-integration]

🔴 UnicodeDecodeError when _render_for_sample receives raw bytes from model_dump()

The code assumes model_dump() on Pydantic's Base64Bytes produces base64-encoded bytes, but it actually produces decoded raw bytes (e.g., raw JPEG binary). The production caller _sample_with_remote_client at skyrl/backends/skyrl_train_backend.py:735 does model_input.model_dump(), which yields {"data": b'\xff\xd8\xff...'} (raw image bytes). When _render_for_sample then calls raw.decode("ascii") on these raw bytes, it raises UnicodeDecodeError.

Comparison with correct pattern in VLLMRenderer

The existing VLLMRenderer._render_images at skyrl/backends/renderer.py:133 handles this correctly:

b64_data = base64.b64encode(chunk.data).decode("ascii")

It first base64-encodes the raw bytes, then decodes to an ASCII string. The fix here should follow the same pattern.

Suggested change

	raw = c["data"]
	b64_str = raw.decode("ascii") if isinstance(raw, bytes) else raw
	raw = c["data"]
	b64_str = base64.b64encode(raw).decode("ascii") if isinstance(raw, bytes) else raw

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