Rename files in sgl kernel to avoid nested folder structure (#4213)

Co-authored-by: zhyncs <me@zhyncs.com>

sgl-kernel/tests/test_trt_allreduce.py

@@ -7,7 +7,7 @@ import unittest
 from typing import Any, List, Optional
 
 import ray
-import sgl_kernel.ops.allreduce as custom_ops
+import sgl_kernel.allreduce as custom_ops
 import torch
 import torch.distributed as dist
 from torch.distributed import ProcessGroup