fix: parse out sse events to solve json decoding error

[ishandhanani]

Motivation

When I run vllm bencher against dynamo ToT I get

File "/usr/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 691, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/sgl-workspace/dynamo/components/backends/sglang/slurm_jobs/scripts/vllm/benchmark_serving.py", line 645, in benchmark
    raise ValueError(
ValueError: Initial test run failed - Please make sure benchmark arguments are correctly specified. Error: Traceback (most recent call last):
  File "/sgl-workspace/dynamo/components/backends/sglang/slurm_jobs/scripts/vllm/backend_request_func.py", line 386, in async_request_dynamo_completions
    data = json.loads(chunk)
           ^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/decoder.py", line 338, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/json/decoder.py", line 356, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

Did a bit of digging and realized this happens due to an llm_metrics event that we emit

...
chunk_bytes=b'data: {"id":"cmpl-2c202c64-6711-43b0-9700-a979896c7484","choices":[],"created":1761605383,"model":"deepseek-ai/DeepSeek-R1","system_fingerprint":null,"object":"text_completion","usage":{"prompt_tokens":991,"completion_tokens":900,"total_tokens":1891}}'
chunk_bytes=b'event: llm_metrics'
Traceback (most recent call last):
  File "/sgl-workspace/dynamo/components/backends/sglang/slurm_jobs/scripts/vllm/benchmark_serving.py", line 1426, in <module>
    main(args)
  File "/sgl-workspace/dynamo/components/backends/sglang/slurm_jobs/scripts/vllm/benchmark_serving.py", line 1041, in main
    benchmark_result = asyncio.run(
                       ^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/runners.py", line 195, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^

This properly parses it out and fixes it

[IzzyPutterman]

Looks good to me, I'll let Kedar merge