Create AbstractPipeline

[alex-hse-repository]

🚀 Feature Request

Create interface for pipelines

Motivation

All pipelines should have common interface

Proposal

Create class AbstractPipeline:

• inherit it from abc
• it should have abstract method fit:
  def fit(self, ts: TSDataset)
• it should have abstract method forecast:
  def forecast(self, prediction_interval: bool = False, quantiles: Sequence[float] = (0.025, 0.975))
• it should have abstract method backtest(see signature here)
• add description

Test cases

No response

Alternatives

No response

Additional context

No response

Checklist

• I discussed this issue with ETNA Team