Bump to sglang 0.4.6.post4 & unified generate sequences ability between sgl and sgl async (#1577)

### Checklist Before Starting

- [x] Search for similar PR(s).
- Thanks to:
  - close #1558 due to mix of prs
  - close #1449 due to partial fix sgl new version issue
  - close #1300 which is part of current pr
- This pr is co-authored with @ocss884 

### What does this PR do?

> Add one-line overview of what this PR aims to achieve or accomplish. 

- bump sglang to 0.4.6.post4
- unified sglang and sglang_async `generate_sequences` api behavior,
e.g. image support
- fix warning for cuda barrier at start of fsdp_workers

### Checklist Before Submitting

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Guide](https://github.com/volcengine/verl?tab=readme-ov-file#contribution-guide).
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- [x] Add `[BREAKING]` to the PR title if it breaks any API.
- [x] Update the documentation about your changes in the
[docs](https://github.com/volcengine/verl/tree/main/docs).
- [x] Add CI test(s) if necessary.

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Co-authored-by: ocss884 <ocss.lin@gmail.com>

requirements_sglang.txt

@@ -17,5 +17,5 @@ torchdata
 torchvision
 transformers
 wandb
-sglang[all]==0.4.4.post4
+sglang[all]==0.4.6.post4
 torch-memory-saver>=0.0.5