text: Title: [BUG] TimeSeries.from_group_dataframe incompatible with integer timestamps
  Body: **Describe the bug**
  `TimeSeries.from_group_dataframe` should support both timestamp and integer time columns, but it internally converts to `DatetimeIndex` regardless of what is passed.
  
  https://github.com/unit8co/darts/blob/a646adf10fa73d8facb16a611f8a3682dc8a1191/darts/timeseries.py#L869-L871
  
  **To Reproduce**
  ```python
  import pandas as pd
  import darts
  df = pd.DataFrame({
      "group": ["a", "a", "a", "b", "b", "b"],
      "t": [0, 1, 2, 0, 1, 2],
      "x": [1.0, 2.0, 3.0, 2.0, 3.0, 4.0],
  })
  ts = darts.TimeSeries.from_group_dataframe(df, group_cols="group", time_col="t")
  
  ts[0].time_index
  # DatetimeIndex([          '1970-01-01 00:00:00',
  #                '1970-01-01 00:00:00.000000001',
  #                '1970-01-01 00:00:00.000000002'],
  #               dtype='datetime64[ns]', name='t', freq='ns')
  ```
  
  **Expected behavior**
  `time_index` should be a `RangeIndex`.
  
  **System (please complete the following information):**
   - Python version: 3.10
   - darts version 0.30.0
  