Chronos : make is_predict=True works for BaseForecaster

[rnwang04]

Description

This PR makes is_predict=True works for BaseForecaster

1. Why the change?

https://github.com/intel-analytics/BigDL/issues/5874

2. User API changes

Before API is :

# BaseForecaster
x, y = tsdata_test.roll(lookback=48, horizon=0).to_numpy()
# AutoformerForecaster
x, y, x_enc, y_enc = tsdata_test.roll(lookback=48, horizon=1, is_predict=True, time_enc=True).to_numpy()

Now :

# BaseForecaster
x = tsdata_test.roll(lookback=48, horizon=1, is_predict=True).to_numpy()
# AutoformerForecaster
x, y, x_enc, y_enc = tsdata_test.roll(lookback=48, horizon=1, is_predict=True, time_enc=True).to_numpy()

3. Summary of the change

4. How to test?

• [ ] Unit test
• [ ] Application test