Set sliding window to null to match Mistral-7B-Instruct-v0.2

I think the sliding_window should be set to null to match https://huggingface.co/mistralai/Mistral-7B-Instruct-v0.2/blob/41b61a33a2483885c981aa79e0df6b32407ed873/config.json#L19
If this model was pulled from https://huggingface.co/alpindale/Mistral-7B-v0.2-hf/blob/main/config.json then they also updated their config at a later time to drop the sliding window parameter.

If you intended the sliding window, please feel free to close the PR.

config.json

@@ -17,7 +17,7 @@
   "num_key_value_heads": 8,
   "rms_norm_eps": 1e-05,
   "rope_theta": 1000000.0,
-  "sliding_window": 32768,
+  "sliding_window": null,
   "tie_word_embeddings": false,
   "torch_dtype": "float16",
   "transformers_version": "4.38.0.dev0",