Chronos : Support val data in autoformer fit

[rnwang04]

Description

We need to align function in BaseForecaster and AutoformerForecaster synchronously.

1. Why the change?

https://github.com/intel-analytics/BigDL/issues/5834

2. User API changes

from bigdl.chronos.forecaster import AutoformerForecaster
forecaster = AutoformerForecaster(...)
forecaster.fit(data, validation_data)

3. Summary of the change

• [x] support validation_data, validation_mode and earlystop_patience for AutoformerForecaster.fit
• [x] support get_model for AutoformerForecaster
• [x] code refactor

4. How to test?

• [ ] Unit test
• [ ] Application test