fix: minor fix after #1384 (#1476)

<!-- .github/pull_request_template.md -->

## 📌 Description

<!-- What does this PR do? Briefly describe the changes and why they’re
needed. -->

## 🔍 Related Issues

<!-- Link any related issues here -->

## 🚀 Pull Request Checklist

Thank you for contributing to FlashInfer! Before we review your pull
request, please make sure the following items are complete.

### ✅ Pre-commit Checks

- [ ] I have installed `pre-commit` by running `pip install pre-commit`
(or used your preferred method).
- [ ] I have installed the hooks with `pre-commit install`.
- [ ] I have run the hooks manually with `pre-commit run --all-files`
and fixed any reported issues.

> If you are unsure about how to set up `pre-commit`, see [the
pre-commit documentation](https://pre-commit.com/).

## 🧪 Tests

- [ ] Tests have been added or updated as needed.
- [ ] All tests are passing (`unittest`, etc.).

## Reviewer Notes

<!-- Optional: anything you'd like reviewers to focus on, concerns, etc.
-->

Author: yyihuang

flashinfer/prefill.py

@@ -2096,7 +2096,8 @@ def run(
                 lse=lse,
             )
         else:
-            assert self._plan_info is not None, "plan info is not initialized"
+            if self._backend != "trtllm-gen":
+                assert self._plan_info is not None, "plan info is not initialized"
             run_args = [
                 self._float_workspace_buffer,
                 self._int_workspace_buffer,