Refactor model and conversation template registration

[merrymercy]

• Refactor model registration. Now adding a new model only needs adding two self-contained code blocks without using "if": a conversation template and a model adapter.
• API changes

old	new
from fastchat.conversation import get_default_conv_template	from fastchat import get_conversation_template
from fastchat.serve.inference import load_model, add_model_args	from fastchat import load_model, add_model_args

see also the diff for fastchat/serve/huggingface_api.py

• Other minor improvements