[rollout] fix: sglang megatron backend missing generate function

[wuxibin89]

What does this PR do?

Close #2366

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[gemini-code-assist]

Code Review

This pull request adds a generate function to the sglang megatron backend, addressing issue #2366. It also updates test utilities to support both fsdp and megatron backends.

[wuxibin89]

@chenhaiq sglang seems don't work with megatron param_offload=True

[chenhaiq]

Can you test with this change?
https://github.com/volcengine/verl/pull/2365/files#diff-3ea40b07302076f020ceb946048c8129b9bd9141feebef80312da55c431de3feR155

Looks like we have some duplicated work in:
https://github.com/volcengine/verl/pull/2365/files#diff-336120079b31a71a3ec16d4e8a9429f73d5f367d8fa4f2f9a3d89d62a8f5a503R695

[wuxibin89]

Skip this test temporary since it's only works with vllm==0.7.3, our ci image is vllm==0.8.5.

[wuxibin89]

Close this PR since #2365 already fixed it.