Update chat_template.json to incorporate `generation` tag

Incorporating `generation` tag to chat_template.json so that `return_assistant_tokens_mask` can work correctly with the tokenizer (https://github.com/huggingface/transformers/pull/30650/files). A self-contained test script is pasted below.

```python
from transformers import AutoProcessor

processor = AutoProcessor.from_pretrained("HuggingFaceM4/idefics2-8b")

# Define a chat histiry and use `apply_chat_template` to get correctly formatted prompt
# Each value in "content" has to be a list of dicts with types ("text", "image")
conversation = [
{
"role": "system",
"content": [
{"type": "text", "text": "You are a helpful assistant."},
]
},
{
"role": "user",
"content": [
{"type": "text", "text": "What is shown in this image?"},
{"type": "image"},
],
},
{
"role": "assistant",
"content": [
{"type": "text", "text": "This is a picture of a cat."},
]
},
{
"role": "user",
"content": [
{"type": "text", "text": "What is the cat doing?"},
]
},
{
"role": "assistant",
"content": [
{"type": "text", "text": "The cat is sleeping on a sofa. It looks very comfortable."},
]
}
]

template = (
"{% for message in messages %}"
"{{message['role'].capitalize()}}"
"{% if message['content'][0]['type'] == 'image' %}"
"{{':'}}"
"{% else %}"
"{{': '}}"
"{% endif %}"
"{% if message['role'] != 'assistant' %}"
"{% for line in message['content'] %}"
"{% if line['type'] == 'text' %}"
"{{line['text']}}"
"{% elif line['type'] == 'image' %}"
"{{ '<image>' }}"
"{% endif %}"
"{% endfor %}"
"<end_of_utterance>\n"
"{% else %}"
"{% for line in message['content'] %}"
"{% if line['type'] == 'text' %}"
"{% generation %}"
"{{line['text']}}"
"<end_of_utterance>\n"
"{% endgeneration %}"
"{% endif %}"
"{% endfor %}"
"{% endif %}"
"{% endfor %}"
"{% if add_generation_prompt %}"
"{{ 'Assistant:' }}"
"{% endif %}"
)
print("\n")
print(repr(template))
print("\n")

prompt = processor.apply_chat_template(
conversation,
chat_template=template,
add_generation_prompt=False,
tokenize=False
)
print(prompt)

inputs = processor.apply_chat_template(
conversation,
chat_template=template,
add_generation_prompt=False,
tokenize=True,
return_assistant_tokens_mask=True,
return_dict=True
)
print(inputs['assistant_masks'])
```

processor_config.json

@@ -1,5 +1,5 @@
 {
-  "chat_template": "{% for message in messages %}{{message['role'].capitalize()}}{% if message['content'][0]['type'] == 'image' %}{{':'}}{% else %}{{': '}}{% endif %}{% for line in message['content'] %}{% if line['type'] == 'text' %}{{line['text']}}{% elif line['type'] == 'image' %}{{ '<image>' }}{% endif %}{% endfor %}<end_of_utterance>\n{% endfor %}{% if add_generation_prompt %}{{ 'Assistant:' }}{% endif %}",
+  "chat_template": "{% for message in messages %}{{message['role'].capitalize()}}{% if message['content'][0]['type'] == 'image' %}{{':'}}{% else %}{{': '}}{% endif %}{% if message['role'] != 'assistant' %}{% for line in message['content'] %}{% if line['type'] == 'text' %}{{line['text']}}{% elif line['type'] == 'image' %}{{ '<image>' }}{% endif %}{% endfor %}<end_of_utterance>\n{% else %}{% for line in message['content'] %}{% if line['type'] == 'text' %}{% generation %}{{line['text']}}<end_of_utterance>\n{% endgeneration %}{% endif %}{% endfor %}{% endif %}{% endfor %}{% if add_generation_prompt %}{{ 'Assistant:' }}{% endif %}",
   "image_seq_len": 64,
   "processor_class": "Idefics2Processor"
 }