Add Pydantic v2 support

[mrwyattii]

No description provided.

[ringohoffman]

From the migration guide:

Due to performance overhead and implementation complexity, we have now deprecated support for specifying json_encoders in the model config. This functionality was originally added for the purpose of achieving custom serialization logic, and we think the new serialization decorators are a better choice in most common scenarios.

I think this would be effectively the same as json_encoders={torch.dtype: lambda x: str(x)}:

    @field_serializer('dtype')
    def serialize_dtype(
        self, v: Union[torch.dtype, DtypeEnum], info: SerializationInfo
    ) -> str:
        if isinstance(v, torch.dtype):
            return str(v)
        return str(v.value)

[loadams]

Closing this instance as it is repetitive with the actual pydantic PR that will be merged