[Misc] Add indirection layer for custom ops (#3913)

vllm/model_executor/layers/activation.py

@@ -6,7 +6,7 @@ import torch
 import torch.nn as nn
 import torch.nn.functional as F
 
-from vllm._C import ops
+from vllm import _custom_ops as ops
 from vllm.distributed import (divide, get_tensor_model_parallel_rank,
                               get_tensor_model_parallel_world_size)
 from vllm.model_executor.layers.quantization import QuantizationConfig