fix Pydantic model parsing

[DeNeutoy]

Fixes #1048, #1043

Problem

When parsing json schemas generated from a Pydantic model, if the schema has references to other models, it contains a $defs key, which are referred to in the schema itself via $ref. This $defs key does not have a type key, as it is actually defining the types - and so the SchemaConverter throws an error, as it requires all schema keys to have a type. This assert is actually not required, as it is caught later on after iterating through the various options for schema_types here:

https://github.com/abetlen/llama-cpp-python/compare/main...DeNeutoy:fix-model-parsing?expand=1#diff-22ae5683f6e444008ca0411a78851823be188bd8e7f287327397462c5671a1a4L1502

I've deleted it and added a test. Cool project, thanks!

[abetlen]

@DeNeutoy thank you for the contribution!