Update web_demo.py

1. 实测 `from transformers import AutoModelForCausalLM, AutoTokenizer` 必须先引入 `AutoTokenizer` 再引入`AutoModelForCausalLM` 否则会报以下错

```bash
Traceback (most recent call last):
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/scriptrunner/script_runner.py", line 552, in _run_script
    exec(code, module.__dict__)
  File "/root/code/InternLM2/web_demo.py", line 10, in <module>
    from transformers import AutoModelForCausalLM, AutoTokenizer
ImportError: cannot import name 'AutoModelForCausalLM' from 'transformers' (/root/.local/lib/python3.10/site-packages/transformers/__init__.py)
load model begin.
```

2. 必须用 steamlit 命令，不能用 python 命令运行 web_demo.py 否则会报以下错

``` bash
2024-01-19 15:02:13.807 Session state does not function when running a script without `streamlit run`
Traceback (most recent call last):
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/session_state.py", line 378, in __getitem__
    return self._getitem(widget_id, key)
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/session_state.py", line 423, in _getitem
    raise KeyError
KeyError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/session_state_proxy.py", line 119, in __getattr__
    return self[key]
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/session_state_proxy.py", line 90, in __getitem__
    return get_session_state()[key]
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/safe_session_state.py", line 113, in __getitem__
    return self._state[key]
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/session_state.py", line 380, in __getitem__
    raise KeyError(_missing_key_error_message(key))
KeyError: 'st.session_state has no key "messages". Did you forget to initialize it? More info: https://docs.streamlit.io/library/advanced-features/session-state#initialization'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "//root/code/InternLM2/web_demo.py", line 251, in <module>
    main()
  File "//root/code/InternLM2/web_demo.py", line 214, in main
    for message in st.session_state.messages:
  File "/root/.conda/envs/InternLM/lib/python3.10/site-packages/streamlit/runtime/state/session_state_proxy.py", line 121, in __getattr__
    raise AttributeError(_missing_attr_error_message(key))
AttributeError: st.session_state has no attribute "messages". Did you forget to initialize it? More info: https://docs.streamlit.io/library/advanced-features/session-state#initialization
```

chat/web_demo.py

@@ -5,6 +5,7 @@ Please refer to these links below for more information:
     1. streamlit chat example: https://docs.streamlit.io/knowledge-base/tutorials/build-conversational-apps
     2. chatglm2: https://github.com/THUDM/ChatGLM2-6B
     3. transformers: https://github.com/huggingface/transformers
+Please run with the command `streamlit run path/to/web_demo.py --server.address=0.0.0.0 --server.port 7860` instead of using `python path/to/web_demo.py`.
 """
 
 import copy
@@ -15,7 +16,7 @@ from typing import Callable, List, Optional
 import streamlit as st
 import torch
 from torch import nn
-from transformers import AutoModelForCausalLM, AutoTokenizer
+from transformers import AutoTokenizer, AutoModelForCausalLM
 from transformers.generation.utils import LogitsProcessorList, StoppingCriteriaList
 from transformers.utils import logging