[multi-lora] skip pause/resume for lora with merge=false

[erictang000]

pausing and resuming generation are not needed when updating lora weights in vLLM - we can simply rely on load_lora_adapter (https://docs.vllm.ai/en/stable/features/lora/#in-place-lora-reloading) to directly update the weights in the engine without worrying about corrupted weight state.

Results for fully async RL with megatron + vllm LoRA - we can see that removing the pause/resume for the merge lora = false case still works and has an identical reward curve!

[gemini-code-assist]

Code Review

This pull request adds a training script for fully async GRPO on GSM8K with Megatron and LoRA. It also updates the weight synchronization logic to avoid pausing generation when using non-merged LoRA. Feedback indicates that the condition for skipping the pause is too broad and could cause errors on non-Megatron strategies; a more robust implementation using getattr and a backend check for vLLM is recommended.