Update _update_regressors logic with new transforms behavior

[julia-shenshina]

🚀 Feature Request

In #326 we are going to add some knowledge about ability to convert data to regressors to transforms. In this issue we are need to update TSDataset.transform method according new rules.

Motivation

New way of work with regressors.

Proposal

• delete _update_regressors method
• in transform loop for each transform check:
  • if it is instance of FutureMixin or it's in_column is in self._regressors, add its output_column to self._regressors
  • do nothing otherwise

Note that once added to self._regressors column cannot be moved out there.

Test cases

Update existing cases with new logic

Alternatives

No response

Additional context

is blocked by #389 #385

Checklist

• I discussed this issue with ETNA Team

[julia-shenshina]

is blocked by #326