KeyError in dictionary for pipeline transformation on new data for single series

After extending the STLDecomposer to support multiseries and changing the class variables to dictionaries, there was an error in `pipeline_tests/test_pipeline_utils.py` where the keys of the series do not match for single series. Currently, we access the values by casting it as a list and then indexing, but it should be further investigated to simplify the code block.

```python
if len(y.columns) > 1:
seasonality = self.seasonalities[id]
trend = self.trends[id]
residual = self.residuals[id]
period = self.periods[id]
else:
seasonality = list(self.seasonalities.values())[0]
trend = list(self.trends.values())[0]
residual = list(self.residuals.values())[0]
period = list(self.periods.values())[0]
```