Refactor sliding window configuration to Transformers best practice (#21927)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/contributing/model/basic.md

@@ -117,7 +117,7 @@ For models with interleaving sliding windows (e.g. `google/gemma-2-2b-it` and `m
 
 To support a model with interleaving sliding windows, we need to take care of the following details:
 
-- Make sure the model's `config.json` contains `sliding_window_pattern`. vLLM then sets `self.hf_text_config.interleaved_sliding_window` to the value of `self.hf_text_config.sliding_window` and deletes `sliding_window` from `self.hf_text_config`. The model will then be treated as a full-attention model.
+- Make sure the model's `config.json` contains `layer_types`.
 - In the modeling code, parse the correct sliding window value for every layer, and pass it to the attention layer's `per_layer_sliding_window` argument. For reference, check [this line](https://github.com/vllm-project/vllm/blob/996357e4808ca5eab97d4c97c7d25b3073f46aab/vllm/model_executor/models/llama.py#L171).
 
 With these two steps, interleave sliding windows should work with the model.