Fix unit test

Author: xuzhao9

tritonbench/operators/layer_norm/operator.py

@@ -40,6 +40,7 @@ def torch_compile_layer_norm(self, *args):
             import torch._functorch.config
 
             torch._functorch.config.donated_buffer = False
+        import torch
 
         @torch.compile
         def inner(*args):