input: Write a python function to train a linear regression model using scikit-learn.
  assert isinstance(train_linear_regression(X_train, y_train), LinearRegression)
  assert train_linear_regression(X_train, y_train).coef_ is not None
  assert train_linear_regression(X_train, y_train).intercept_ is not None
output: def train_linear_regression(X_train, y_train):
      from sklearn.linear_model import LinearRegression
      
      model = LinearRegression()
      model.fit(X_train, y_train)
      
      return model