Fix breakage problem when using custom_ar

[kkHuang-amd]

Motivation

Run the moel test and found below error is reported

==============================================
File "/root/sglang-0.4.3post3_car/python/sglang/srt/managers/scheduler.py", line 1887, in run_scheduler_process
    scheduler = Scheduler(server_args, port_args, gpu_id, tp_rank, dp_rank)
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/managers/scheduler.py", line 243, in __init__
    self.tp_worker = TpWorkerClass(
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/managers/tp_worker_overlap_thread.py", line 63, in __init__
    self.worker = TpModelWorker(server_args, gpu_id, tp_rank, dp_rank, nccl_port)
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/managers/tp_worker.py", line 68, in __init__
    self.model_runner = ModelRunner(
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/model_executor/model_runner.py", line 192, in __init__
    min_per_gpu_memory = self.init_torch_distributed()
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/model_executor/model_runner.py", line 268, in init_torch_distributed
    initialize_model_parallel(tensor_model_parallel_size=self.tp_size)
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/distributed/parallel_state.py", line 1116, in initialize_model_parallel
    _TP = init_model_parallel_group(
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/distributed/parallel_state.py", line 943, in init_model_parallel_group
    return GroupCoordinator(
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/distributed/parallel_state.py", line 263, in __init__
    self.ca_comm = CustomAllreduce(
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/distributed/device_communicators/custom_all_reduce.py", line 281, in __init__
    self.meta = ops.allocate_meta_buffer(ops.meta_size() + max_size)
  File "/root/sglang-0.4.3post3_car/python/sglang/srt/_custom_ops.py", line 124, in meta_size
    return sgl_kernel.ops.meta_size()
AttributeError: module 'sgl_kernel.ops' has no attribute 'meta_size'
=====================================================

Check the path in sgl-kernel, ops is separated multiple files from init.py

Modifications

Change _custom_ops.py to specify the correct calling path

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

[HaiShaw]

LG