[Feature Request]:optional ai model during chunk creation

### Self Checks

- [x] I have searched for existing issues [search for existing issues](https://github.com/infiniflow/ragflow/issues), including closed ones.
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- [x] Please do not modify this template :) and fill in all the required fields.

### Is your feature request related to a problem?

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### Describe the feature you'd like

Allows to customize the ai model instead of directly using the default


### Describe implementation you've considered

_No response_

### Documentation, adoption, use case

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### Additional information

_No response_

The request does not name any files, tests, or entry points. Start by locating chunk creation and the default AI model selection, then determine how an optional custom model would be configured; done means chunk creation can use a chosen model instead of always using the default.

Written by the indexing model from the issue text.