[Fix] error when processing batch prompts for VLM

[JustinTong0323]

Motivation

Previously, the main branch would throw AssertionError when handling batch prompts due to redundant assert.

Modifications

As titled.

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

[JustinTong0323]

@mickqian Could you please have a look?
cc @zhaochenyang20 I think it's high priority.