Add negative_prompt parameter to GLMImagePipeline

[akshan-main]

What does this PR do?

Fixes #13421

Adds negative_prompt parameter to GLMImagePipeline.__call__() and encode_prompt(). Previously, the unconditional prompt was hardcoded to "" when CFG was active, with no way for users to provide a custom negative prompt.

Discussed in #13416

Changes:

• Added negative_prompt: str | list[str] | None = None parameter to __call__() and encode_prompt()
• Falls back to "" when None (backward compatible)
• Added length validation when negative_prompt is a list
• Updated docstrings

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• Was this discussed/approved via a GitHub issue or the forum? Add negative_prompt parameter to GLMImagePipeline #13421
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• Did you write any new necessary tests?

Who can review?

@asomoza @sayakpaul @yiyixuxu

[yiyixuxu]

hi @akshan-main

Thanks for the PR, however, I think the parameter was not exposed by design because this pipeline does not accept negative_prompt - the model was not trained to work with a custom negative prompt