Add support for llava-1-5-phi3-mini

llava-phi-3-mini uses the Phi-3-instruct chat template. I think is similar with current llava-1-5, but with Phi3 instruct template instead of llama 2.

format:
`<|user|>\nQuestion <|end|>\n<|assistant|>`
stop word is <|end|>
for system use: <|system|>

I think you can adapt easily llava-1-5 handler for phi3:
```
class Llava15ChatHandler:
DEFAULT_SYSTEM_MESSAGE: Optional[str] = "A chat between a curious human and an artificial intelligence assistant. The assistant gives helpful, detailed, and polite answers to the human's questions."

CHAT_FORMAT = (
"{% for message in messages %}"
"{% if message.role == 'system' %}"
"{{ message.content }}"
"{% endif %}"
"{% if message.role == 'user' %}"
"{% if message.content is string %}"
"\nUSER: {{ message.content }}"
"{% endif %}"
"{% if message.content is iterable %}"
"\nUSER: "

"{% for content in message.content %}"
"{% if content.type == 'image_url' and content.image_url is string %}"
"{{ content.image_url }}"
"{% endif %}"
"{% if content.type == 'image_url' and content.image_url is mapping %}"
"{{ content.image_url.url }}"
"{% endif %}"
"{% endfor %}"

"{% for content in message.content %}"
"{% if content.type == 'text' %}"
"{{ content.text }}"
"{% endif %}"
"{% endfor %}"

"{% endif %}"
"{% endif %}"
"{% if message.role == 'assistant' and message.content is not none %}"
"\nASSISTANT: {{ message.content }}"
"{% endif %}"
"{% endfor %}"
"{% if add_generation_prompt %}"
"\nASSISTANT: "
"{% endif %}"
)

```

Take a look: @abetlen