[feature] Add more data to train api testing notebook.

[biranyucel]

Define the datasets the same way as the format in the notebook. You can use the datasets under the datasets folder.

 testing_datasets = [
    {
        "file_name": "https://github.com/ourownstory/neuralprophet-data/raw/main/datasets/air_passengers.csv",
        "format": "csv",
        "time_col_index": 0,
        "target_col_index": -1,
        "exovar": False,
    },
    {
        "file_name": "../datasets/air_quality/BeijingPM25_0.csv",
        "format": "csv",
        "time_col_index": 0,
        "target_col_index": -1,
        "exovar": False,
    }
]

Update prepare_dataset if needed.