TimeSeriesSplit add skip parameter

### Describe the workflow you want to enable

Dear sklearn community, I want to make an hour ahead forecast on a timeseries of at least a year with an interval of 5 minutes. To do CV, I can use TimeSeriesSplit with test_size = 12. I want to do this for many different times of the day, so I could use n_splits = 24 and have a split for each hour of the day. Then I want this for multiple days, so I could do n_splits = 24 * nr_days. My problem is that I want to randomly test a few hours a day over many days of the year, to see how my forecast does on different periods of the year. To then make n_splits = 24 * 365 is too many splits.

### Describe your proposed solution

I propose a `skip` parameter, that allows skipping a number of samples before making the next split. This can be used to reduce the number of splits, while still allowing to cover a large time-period with splits.

A few examples:
Data:
| index      | foo |
|------------|-----|
| 2022-01-01 | 1   |
| 2022-01-02 | 2   |
| 2022-01-03 | 3   |
| 2022-01-04 | 4   |
| 2022-01-05 | 5   |
| 2022-01-06 | 6   |
| 2022-01-07 | 7   |
| 2022-01-08 | 8   |
| 2022-01-09 | 9   |
| 2022-01-10 | 10 |

test_size=1, max_train_size=1, n_splits=3, gap=1, skip=1

| train | test |
|-------|-----|
| 8     | 10    |
| 6     | 8      |
| 4     | 6      |

test_size=1, max_train_size=1,   n_splits=3, gap=1, skip=2
| train | test |
|-------|-----|
| 8     | 10   |
| 5     | 7    |
| 2     | 4    |

test_size=2, max_train_size=1, n_splits=3, gap=1, skip=2
not enough data

test_size=2, max_train_size=1, n_splits=2, gap=1, skip=2
| train | test  |
|-------|------|
| 7     | 9, 10 |
| 3     | 5, 6  |

test_size=2, max_train_size=1, n_splits=2, gap=1, skip=2
| train | test  |
|-------|------|
| 7     | 9, 10 |
| 3     | 5, 6  |

test_size=2, n_splits=2, gap=2, skip=2
| train               | test  |
|----------------|------|
| 1, 2                | 5, 6  |
| 1, 2, 3, 4, 5, 6 | 9, 10 |

### Describe alternatives you've considered, if relevant

I could generate all those splits, then implement a wrapper in my own codebase that does the skipping.

### Additional context

I already have this solution implemented locally. The important changes to the function `TimeSeriesSplit.split()` are:

```
        if n_samples - gap - ((test_size + skip) * n_splits) < 0:
            raise ValueError(
                f"Too many splits={n_splits} for number of samples"
                f"={n_samples} with test_size={test_size}, gap={gap} and skip={skip}."
            )
```

```
test_starts = range(n_samples - n_splits * (test_size + skip) + skip, n_samples, test_size + skip)
```

I'd be happy to make a PR if you think this can be included. Thank you for your time and consideration!

COMMENT:
Hey @glemaitre , that's not possible as far as I can tell. `gap` is for in between the train and test part of a split, my suggested `skip` is for in between whole splits. The code where `gap` is used is

```
for test_start in test_starts:
    train_end = test_start - gap
```

Labels: New Feature, module:model_selection, Needs Decision - Include Feature