[doc] add missing imports (#15699)

Signed-off-by: reidliu41 <reid201711@gmail.com>
Co-authored-by: reidliu41 <reid201711@gmail.com>

docs/source/serving/offline_inference.md

@@ -11,6 +11,8 @@ For example, the following code downloads the [`facebook/opt-125m`](https://hugg
 and runs it in vLLM using the default configuration.
 
 ```python
+from vllm import LLM
+
 llm = LLM(model="facebook/opt-125m")
 ```
 
@@ -47,6 +49,8 @@ To fix this, explicitly specify the model architecture by passing `config.json`
 For example:
 
 ```python
+from vllm import LLM
+
 model = LLM(
     model="cerebras/Cerebras-GPT-1.3B",
     hf_overrides={"architectures": ["GPT2LMHeadModel"]},  # GPT-2
@@ -92,6 +96,8 @@ You can further reduce memory usage by limiting the context length of the model
 and the maximum batch size (`max_num_seqs` option).
 
 ```python
+from vllm import LLM
+
 llm = LLM(model="adept/fuyu-8b",
           max_model_len=2048,
           max_num_seqs=2)