fix: restore CPU flash_attn test to use sgl_kernel directly (#22573)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/srt/cpu/test_flash_attn.py

@@ -1,12 +1,14 @@
 import unittest
 
+import sgl_kernel  # noqa: F401
 import torch
 import torch.nn.functional as F
 from utils import parametrize, precision
 
-from sglang.jit_kernel.flash_attention import flash_attn_varlen_func
 from sglang.test.test_utils import CustomTestCase
 
+flash_attn_varlen_func = torch.ops.sgl_kernel.flash_attn_varlen_func
+
 torch.manual_seed(1234)