enable multi-step forecasting with LSTM for both univariate and multivariate

[shanyu-sys]

reference: https://machinelearningmastery.com/how-to-develop-lstm-models-for-multi-step-time-series-forecasting-of-household-power-consumption/

[shane-huang]

Do we support multivariate already? LSTM is a very frequently used model so we'd better enable multivariate and multi step.