HalvingGridSearchCV giving non-optimal results with min_resources='exhaust'

### Describe the bug

I am using `HalvingGridSearchCV` with 160 combinations and 1050 samples. When I use `min_resources` with `'exhaust'`, I get 6 iterations with the last iteration including 5 candidates and 640 samples. The process starts with 20 samples.

When I set `min_resources=30`, I also get 6 iterations with the last iteration having 5 candidates, but with 960 samples.

### Steps/Code to Reproduce

```python
halving_cv = HalvingGridSearchCV(
    knn_model, knn_grid,  #our model and the parameter combos we want to try
    scoring="roc_auc",  #could alternatively choose f1, accuracy or others
    n_jobs=-1,  #use all available cpus
    min_resources=30,  #"exhaust" sets this to 20, which is non-optimal. Possible bug in algorithm.
    factor=2,  #double samples and take top half of combos on each iteration
    cv=5, random_state=1234,
    refit=True,  #remembers the best combo and gives us back that model already trained and ready for testing
)

grid_result = halving_cv.fit(x_train, y_train)
```

### Expected Results

My understanding is that by using 'exhaust', the algorithm should figure out that 30 is a better value for min_resources. Instead it sets it to 20.

My ideal case would be for the last iteration to use all 1050 samples with the 5 candidates but I see no way to set parameters to make this happen.

My question is whether there is a bug in the min_resources=exhaust algorithm.

### Actual Results

With min_resources=30:
```
310	5	960	{'algorithm': 'auto', 'n_neighbors': 15, 'p': 1, 'weights': 'uniform'}	0.804883
311	5	960	{'algorithm': 'brute', 'n_neighbors': 5, 'p': 1, 'weights': 'uniform'}	0.793407
312	5	960	{'algorithm': 'ball_tree', 'n_neighbors': 5, 'p': 1, 'weights': 'uniform'}	0.789650
313	5	960	{'algorithm': 'auto', 'n_neighbors': 5, 'p': 1, 'weights': 'uniform'}	0.789410
314	5	960	{'algorithm': 'kd_tree', 'n_neighbors': 5, 'p': 1, 'weights': 'uniform'}	0.789410
```

### Versions

```shell
System:
    python: 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0]
executable: /usr/bin/python3
   machine: Linux-5.15.109+-x86_64-with-glibc2.35

Python dependencies:
      sklearn: 1.2.2
          pip: 23.1.2
   setuptools: 67.7.2
        numpy: 1.23.5
        scipy: 1.11.2
       Cython: 3.0.2
       pandas: 1.5.3
   matplotlib: 3.7.1
       joblib: 1.3.2
threadpoolctl: 3.2.0

Built with OpenMP: True

threadpoolctl info:
       user_api: blas
   internal_api: openblas
    num_threads: 2
         prefix: libopenblas
       filepath: /usr/local/lib/python3.10/dist-packages/numpy.libs/libopenblas64_p-r0-742d56dc.3.20.so
        version: 0.3.20
threading_layer: pthreads
   architecture: Zen

       user_api: openmp
   internal_api: openmp
    num_threads: 2
         prefix: libgomp
       filepath: /usr/local/lib/python3.10/dist-packages/scikit_learn.libs/libgomp-a34b3233.so.1.0.0
        version: None

       user_api: blas
   internal_api: openblas
    num_threads: 2
         prefix: libopenblas
       filepath: /usr/local/lib/python3.10/dist-packages/scipy.libs/libopenblasp-r0-23e5df77.3.21.dev.so
        version: 0.3.21.dev
threading_layer: pthreads
   architecture: Zen

       user_api: blas
   internal_api: openblas
    num_threads: 2
         prefix: libopenblas
       filepath: /usr/local/lib/python3.10/dist-packages/cvxopt.libs/libopenblasp-r0-5c2b7639.3.23.so
        version: 0.3.23
threading_layer: pthreads
   architecture: Zen
```
```

COMMENT:
> > My understanding is that by using 'exhaust', the algorithm should figure out that 30 is a better value for min_resources. Instead it sets it to 20.
> 
> How do you get "30"? I find 32 when dividing by 2.
> 
> ```python
> >>> 1050 // 2 // 2 // 2 // 2 // 2
> 32
> ```
> 
> but then, why would there be 6 doubling iterations rather than, say, 5...

I just tried 30 as a start. Did not do real search. Point is that default is 20 and that seems wrong.

Labels: Needs Decision