Hi @YaoLin and welcome to the challenge 
!
That’s why we’re here 
About the imports 
You need only define your imports in the specified area in the notebook and it should work fine and be imported throughout. Though multiple imports won’t hurt anyway 
About errors in general 
Generally for submissions we provide the full traceback of the error on your training data. In your case you can see the link to the error here, where it says “[generate-predictions]”:
Your specific error 
In your specific case if you click on that link you will find the following which says that you should be using the native save and load functions for xgboost to save your model. Now if you are saving multiple models then please see this discussion on how to do that.
Traceback (most recent call last):
File "predict.py", line 30, in <module>
model = load_model(submission_config["model_path"])
File "/home/aicrowd/load_model.py", line 7, in load_model
return pickle.load(target)
File "/usr/local/lib/python3.8/site-packages/xgboost/core.py", line 1087, in __setstate__
_check_call(
File "/usr/local/lib/python3.8/site-packages/xgboost/core.py", line 189, in _check_call
raise XGBoostError(py_str(_LIB.XGBGetLastError()))
xgboost.core.XGBoostError: [17:32:31] ../src/learner.cc:922: Check failed: header == serialisation_header_:
If you are loading a serialized model (like pickle in Python) generated by older
XGBoost, please export the model by calling `Booster.save_model` from that version
first, then load it back in current version. There's a simple script for helping
the process. See:
https://xgboost.readthedocs.io/en/latest/tutorials/saving_model.html
for reference to the script, and more details about differences between saving model and
serializing.
I hope this helps 