[smollm3] add tokenizer mapping for smollm3

[gante]

What does this PR do?

Adds a tokenizer mapping for smollm3, which fixes the following script

from transformers import pipeline

# tokenizer is infered from the model, through the tokenizer mapping
pipe = pipeline("text-generation", model="HuggingFaceTB/SmolLM3-3B-Base")
pipe("Once upon a time,")

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: auto

[HuggingFaceDocBuilderDev]

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