Creating multiple InferenceModels with the same ModelName causes inconsistencies

[ahg-g]

Currently the logic tracks the models by Spec.ModelName, since this is not guaranteed to be unique within the cluster, we could run into two issues:

1. If the model name changes on the same InferenceModel object, we don't delete the original model entry in the datastore.
2. We don't enforce the semantics that the modelName with the oldest creation timestamp is retained. While the api is assuming that this is enforced by another controller via the Ready condition, we don't have this controller yet, and so currently the behavior is unpredictable depending on InferenceModel events order.

/kind bug