Putting back cudnn_batch_prefill_with_kv_cache that was deleted by ruff (flashinfer-ai#1438)

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

## 📌 Description

Adds back cudnn_batch_prefill_with_kv_cache in prefill.py that was deleted due to ruff.

<!-- 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

- [x] I have installed `pre-commit` by running `pip install pre-commit`
(or used your preferred method).
- [x] I have installed the hooks with `pre-commit install`.
- [x] 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

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

## Reviewer Notes

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

Author: bkryu

flashinfer/prefill.py

@@ -22,6 +22,9 @@
 
 import torch
 
+from .cudnn import (
+    cudnn_batch_prefill_with_kv_cache as cudnn_batch_prefill_with_kv_cache,
+)
 from .jit import (
     gen_batch_prefill_module,
     gen_customize_batch_prefill_module,