Add `eos_token` to the tokenizer config.

If we merge the ChatWidget right now, it would not work since the widget would not be able to format the chat_template correctly from the information available via API (https://huggingface.co/api/models/microsoft/DialoGPT-large?config=True). This is because to get `eos_token` one need to get `eos_token_id` from [config.json](https://huggingface.co/microsoft/DialoGPT-large/blob/main/config.json) and then reading [`vocab.json`](https://huggingface.co/microsoft/DialoGPT-large/blob/main/vocab.json) to check which token is associated with this id.

This PR fixes this by adding `eos_token` directly to `tokenizer_config.json` cc

@julien-c


@osanseviero


@sbrandeis

tokenizer_config.json

@@ -1,4 +1,5 @@
 {
   "model_max_length": 1024,
-  "chat_template": "{% for message in messages %}{{ message.content }}{{ eos_token }}{% endfor %}"
+  "chat_template": "{% for message in messages %}{{ message.content }}{{ eos_token }}{% endfor %}",
+  "eos_token": "<|endoftext|>"
 }