add sentencepiece to requirements.txt for SD3 dreambooth

[jorahn]

add sentencepiece to requirements.txt for SD3 dreambooth

What does this PR do?

sd3 dreambooth requires sentencepiece but it's not in the requirements.

Fixes # (issue)

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• Did you write any new necessary tests?

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@sayakpaul

[sayakpaul]

Could you resolve the conflicts?

[jorahn]

Could you resolve the conflicts?

Let me check!

[jorahn]

@sayakpaul done

[sayakpaul]

@jorahn apologies for the trouble but could you push an empty commit to trigger the CI?

[jorahn]

@sayakpaul do you need anything else? Don't think the build errors are related.

[sayakpaul]

Thank you!