Fix regression

Mixtral doesn't use sliding window attention. We force set it to null since the default in transformers is 4k.
I think the regression came from https://huggingface.co/mistralai/Mixtral-8x7B-Instruct-v0.1/commit/858fdc292793fc3e671bf51fc5586c5cc10fbe3a .

@ybelkada
did you use a specific script to update the PR? If so I think you need to update that script so that the default write "null".

config.json

@@ -20,7 +20,7 @@
   "rms_norm_eps": 1e-05,
   "rope_theta": 1000000.0,
   "router_aux_loss_coef": 0.02,
-  "sliding_window": 4096,
+  "sliding_window": null,
   "tie_word_embeddings": false,
   "torch_dtype": "bfloat16",
   "transformers_version": "4.36.0.dev0",