Fix is_multimodal_model judge

[hnyls2002]

if isinstance(model, str):
    return "llava" or "yi-vl" in model

The above logic is wrong, when model is llama-7b, it returns "llava" but not True or False.

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/ubuntu/sglang/python/sglang/launch_server.py", line 11, in <module>
    launch_server(server_args, None)
  File "/home/ubuntu/sglang/python/sglang/srt/server.py", line 356, in launch_server
    tokenizer_manager = TokenizerManager(server_args, port_args)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/ubuntu/sglang/python/sglang/srt/managers/tokenizer_manager.py", line 106, in __init__
    self.tokenizer = self.processor.tokenizer
                     ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'LlamaTokenizerFast' object has no attribute 'tokenizer'. Did you mean: '_tokenizer'?