plot_forecast should work without test_ts

[iKintosh]

🐛 Bug Report

Now plot_forecast requires test_ts argument. But sometimes I want to plot only train data and forecast. So we should make test_ts argument optional.

Expected behavior

User should be able to plot only forecast, forecast + train_ts, forecast + test_ts and forecast + train_ts + test_ts.

How To Reproduce

Code

etna/etna/analysis/plotters.py

Lines 21 to 28 in e66058f

	def plot_forecast(
	forecast_ts: "TSDataset",
	test_ts: "TSDataset",
	train_ts: Optional["TSDataset"] = None,
	segments: Optional[List[str]] = None,
	n_train_samples: Optional[int] = None,
	columns_num: int = 2,
	):

Environment

Any, etna 1.3.3

Additional context

No response

Checklist

• Bug appears at the latest library version