fix: prefer serde structs over custom functions (#2127)
* fix: prefer enum for chat object
* fix: adjust typo
* fix: enum CompletionType not ObjectType
* fix: adjust typo
* feat: leverage serde for conditional deser
* fix: adjust HubTokenizerConfig after rebase
* fix: update create_post_processor logic for token type
* fix: adjust unwrap syntax in template
* Fixing the post processor.
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Co-authored-by: Nicolas Patry <patry.nicolas@protonmail.com>