Change ProphetModel and SARIMAXModel according to latest architecture

[iKintosh]

🚀 Feature Request

Both should inherit from PerSegmentPredictionIntervalModel.

Motivation

Make final changes for PredictionIntervalModels

Proposal

1. Neither ProphetModel, nor SARIMAXModel should implement forecast and forecast_one_segment methods after this task.

• Rename _ProphetModel to _ProphetAdapter
• Rename _SARIMAXModel to _SARIMAXAdapter
• All additional work that is done in forecast and forecast_one_segment methods should be moved either to Adapters or PerSegmentPredictionIntervalModel

2. Change this line, it should check isinstance, not signature.
   

   etna/etna/pipeline/pipeline.py

   Line 156 in 1a10b25

   if "prediction_interval" in inspect.signature(self.model.forecast).parameters:

3. Remove unnecessary methods and parameters from PerSegmentBaseModel:

• Method _build_models
• Parameter _segments

Test cases

No response

Alternatives

Blocked by #506, #508

Additional context

No response

Checklist

• I discussed this issue with ETNA Team