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 LlavaNextVideoProcessor

processor = LlavaNextVideoProcessor.from_pretrained("llava-hf/LLaVA-NeXT-Video-7B-hf")

# 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 video?"},
{"type": "video"},
],
},
{
"role": "assistant",
"content": [
{"type": "text", "text": "This is a video about 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 %}"
"{% if message['role'] != 'system' %}"
"{{ message['role'].upper() + ': '}}"
"{% endif %}"
"{# Render all images first #}"
"{% for content in message['content'] | selectattr('type', 'equalto', 'image') %}"
"{{ '<image>\n' }}"
"{% endfor %}"
"{# Render all images first #}"
"{% for content in message['content'] | selectattr('type', 'equalto', 'video') %}"
"{{ '<video>\n' }}"
"{% endfor %}"
"{# Render all text next #}"
"{% if message['role'] != 'assistant' %}"
"{% for content in message['content'] | selectattr('type', 'equalto', 'text') %}"
"{{ content['text'] + ' '}}"
"{% endfor %}"
"{% else %}"
"{% for content in message['content'] | selectattr('type', 'equalto', 'text') %}"
"{% generation %}"
"{{ content['text'] + ' '}}"
"{% endgeneration %}"
"{% 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'])
```

chat_template.json

@@ -1,3 +1,3 @@
 {
-  "chat_template": "{% for message in messages %}{% if message['role'] != 'system' %}{{ message['role'].upper() + ': '}}{% endif %}{# Render all images first #}{% for content in message['content'] | selectattr('type', 'equalto', 'image') %}{{ '<image>\n' }}{% endfor %}{# Render all images first #}{% for content in message['content'] | selectattr('type', 'equalto', 'video') %}{{ '<video>\n' }}{% endfor %}{# Render all text next #}{% for content in message['content'] | selectattr('type', 'equalto', 'text') %}{{ content['text'] + ' '}}{% endfor %}{% endfor %}{% if add_generation_prompt %}{{ 'ASSISTANT:' }}{% endif %}"
+  "chat_template": "{% for message in messages %}{% if message['role'] != 'system' %}{{ message['role'].upper() + ': '}}{% endif %}{# Render all images first #}{% for content in message['content'] | selectattr('type', 'equalto', 'image') %}{{ '<image>\n' }}{% endfor %}{# Render all images first #}{% for content in message['content'] | selectattr('type', 'equalto', 'video') %}{{ '<video>\n' }}{% endfor %}{# Render all text next #}{% if message['role'] != 'assistant' %}{% for content in message['content'] | selectattr('type', 'equalto', 'text') %}{{ content['text'] + ' '}}{% endfor %}{% else %}{% for content in message['content'] | selectattr('type', 'equalto', 'text') %}{% generation %}{{ content['text'] + ' '}}{% endgeneration %}{% endfor %}{% endif %}{% endfor %}{% if add_generation_prompt %}{{ 'ASSISTANT:' }}{% endif %}"
 }