Add padding token to config (fix batched generation)

Fixes `Cannot handle batch sizes > 1 if no padding token is defined`

tested with:
```
infinity_emb v2 --model-id michaelfeil/mxbai-rerank-large-v2-seq --device mps --engine torch --no-model-warmup --revision refs/pr/1
```

config.json

@@ -6,6 +6,7 @@
   "attention_dropout": 0.0,
   "bos_token_id": 151643,
   "eos_token_id": 151645,
+  "pad_token_id": 151643,
   "hidden_act": "silu",
   "hidden_size": 896,
   "id2label": {