[Feature] Update Transformers to v5

[ji-huazhong]

Feature request

This RFC proposes upgrading the transformers library dependency from version 4.x to the recently released v5.0.0 across the Verl infrastructure stack. The upgrade is motivated by new features and breaking changes that require coordinated updates across multiple components.

Motivation

https://github.com/huggingface/transformers/releases/tag/v5.0.0

Prerequisites

The upgrade must be performed in a specific order due to dependency chains:

Phase 1: Foundation Layer Updates

1. Rollout Engines must support transformers v5 first:

• vLLM: Update to transformers v5 vllm-project/vllm#30566
• SGLang: [Feature] Update transformers to v5 sgl-project/sglang#17779
• TensorRT-LLM: [Feature]: Migrate to Transformers v5 NVIDIA/TensorRT-LLM#11033

2. Model engines:

• Megatron-Bridge: Update model conversion utilities and checkpoint compatibility layers

Phase 2: Application Layer Updates

3. Verl Core:

• Update model registry and loader
• Adapt to new configuration APIs
• Test integrations