feat(grammar): mark grammar disabled by default

[mudler]

Description

This PR disable grammar by default and make that opt-in. More and more models now support function calling, and grammar support is better to be enabled on these model that fits this logic, or alternatively enabled when want to constrain models that doesn't support function calls directly.

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[localai-bot]

This PR diff introduces a few changes:

1. In core/http/endpoints/openai/chat.go, there is a change in the condition for using a grammar function. Previously, the condition was !config.FunctionsConfig.GrammarConfig.NoGrammar && shouldUseFn. Now, the condition has been updated to config.FunctionsConfig.GrammarConfig.EnableGrammar && shouldUseFn. This change allows the grammar parsing to be enabled or disabled based on the EnableGrammar field in the configuration.

2. In the gallery/mudler.yaml file, there is an addition of the grammar field under the function section. The grammar field has a sub-field enable which is set to true. This enables the grammar parsing for the Mudler gallery.

3. In pkg/functions/parse.go, there is a change in the GrammarConfig struct. The NoGrammar field has been renamed to EnableGrammar, and its type has been updated to a boolean. This change aligns with the changes made in the chat.go file.

These changes seem to be related to enabling or disabling grammar parsing in the OpenAI chat functionality. The logic for grammar parsing has been updated to consider an EnableGrammar boolean flag in the configuration, allowing the feature to be toggled on or off depending on the specific use-case.