KEP-2839: Kubeflow Dynamic LLM Trainer Framework

### What you would like to be added?

I would like to add Kubeflow Dynamic LLM Trainer Framework to:

1. Streamline LLM Fine-tuning on Kubernetes with a simple yet efficient Python SDK
2. Allow dynamic and extensible registration of popular post-training frameworks
3. Make it possible for day0&day1 model support

Google Doc: http://bit.ly/4nyrQS8 (work in progress)

Upstream KEP: https://github.com/kubeflow/trainer/issues/2401

Appreciate your feedbacks! @kubeflow/kubeflow-trainer-team @kubeflow/kubeflow-sdk-team @deepanker13 @saileshd1402 @seanlaii @helenxie-bit @varshaprasad96 @franciscojavierarceo @thesuperzapper @rimolive @juliusvonkohout @jbottum @varodrig @Doris-xm

### Why is this needed?

Coming from https://github.com/kubeflow/trainer/issues/2752, we decided to adopt the plan 3 after several rounds of discussions.

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