Nondaily time series breaks

[AzulGarza]

Using non-daily time series throws the following error:

The minimum reproducible example would be,

import numpy as np
import pandas as pd

from statsforecast import StatsForecast
from statsforecast.models import random_walk_with_drift

rng = np.random.RandomState(0)
serie1 = np.arange(1, 8)[np.arange(100) % 7] + rng.randint(-1, 2, size=100)
serie2 = np.arange(100) + rng.rand(100)
series = pd.DataFrame(
    {
        'ds': pd.date_range('2000-01-01', periods=serie1.size + serie2.size, freq='M'),
        'y': np.hstack([serie1, serie2]),
    },
    index=pd.Index([0] * serie1.size + [1] * serie2.size, name='unique_id')
)

fcst = StatsForecast(series, models=[random_walk_with_drift], freq='M')
forecasts = fcst.forecast(5)                                       

I think the problem might be solved using pd.DatetimeIndex on self.last_dates. I'll open a PR soon.