Implement ability to define splits in metadata section of dataset card

### Feature request

If you go here: https://huggingface.co/datasets/inria-soda/tabular-benchmark/tree/main you will see bunch of folders that has various CSV files. I’d like dataset viewer to show these files instead of only one dataset like it currently does. (and also people to be able to load them as splits instead of loading through `data_files`) 
e.g GLUE has various splits on viewer but it’s too overkill to ask people to implement loading script, so it would be better to let them define these in the README file instead.

Also pinging @polinaeterna @lhoestq @adrinjalali 


Comments:
@merveenoyan Do you want different files to be splits or configurations?

From [what you specified in `Readme.md`](https://huggingface.co/datasets/inria-soda/tabular-benchmark/commit/fb4575853772c62a20203bdd6cc0202f5db4ce4e) I hypothesize that you want to have 4 **configs** corresponding to directories: `"clf_cat", "clf_num", "reg_cat", "reg_num"`. And inside each config you require to have as many splits as there are `csv` files
so if you run 
```python
load_dataset("inria-soda/tabular-benchmark", "clf_cat", split="compass")
```
you will generate the data only from `compass.csv` file.
In this case, running `load_dataset("inria-soda/tabular-benchmark", "clf_cat"`) without split parameter will return `DatasetDict` object with `"KDDCup09_upselling", "cat_compass", "cat_covertype", ... "road_safety"` keys (which values are splits - `Dataset` objects)

**or**
do you want each file to be a separate config? Like:
```python
load_dataset("inria-soda/tabular-benchmark", "clf_cat_compass")   # returns DatasetDict with a single "train" split
```
**or**
maybe smth completely different? :smile:  

Anyway, now I have an impression that this is probably rather a matter of automatically inferring configs from repository structure rather than providing parameters in metadata yaml.


@polinaeterna I want the latter where you can think of every CSV file as a config, like MNLI from GLUE.

@merveenoyan @lhoestq I see two solutions to this case.  
1. Parse configurations automatically from directories names. That is, if you have data structure like:
```
tabular-benchmark
  └─clf_cat_compass
      └─compass.csv
  └─clf_cat_cat_covertype
      └─covertype.csv
 ...
  └─reg_cat_house_sales
      └─house_sales.csv
```
you'll get  "clf_cat_compass", "clf_cat_cat_covertype", ... "reg_cat_house_sales" configurations that would contain **only files from corresponding directories**. 
**\+** this is a requested change and needed in general and would solve other problems, see https://github.com/huggingface/datasets/issues/4578, would also help with https://github.com/huggingface/datasets/pull/5213 which I'm working on currently
**\+** would allow users to do just `load_dataset(“inria-soda/tabular-benchmark”, “clf_cat_compass”)`, no `data_files` param required
**\-** in this specific case it would require restructuring of the data - putting each file in a directory named as a config name (to me personally it doesn't seem to be a big deal) 

2. More or less what we discussed before - add support for manually specifying parameters in the metadata. We can add new metadata yaml field (say, `"custom_configs_info"`), so that we can provide smth like:
```yaml
---
...
dataset_info:
 ...  
custom_configs_info:
- config_name: reg_cat_house_sales
 data_files:
 - reg_cat/house_sales.csv
- config_name: clf_cat_compass
 data_files:
 - clf_cat/compass.csv
...
---
```
**\+** Would be useful not only for tabular data and not only for `data_files` parameter - any packaged dataset’s viewer can be customized to use specific, non-default parameters. @merveenoyan do you maybe have any other examples/use cases in mind where you want to provide any specific parameters to the viewer? 
**\-** I'm not sure here but assume that it might require changes in interaction with the viewer on the hub side - to parse these configurations, as they not default configurations (not in `BUILDER_CONFIGS` list). cc @severo But probably this can be solved on the `datasets` side too.

Overall, I would start from implementing the first solution since it's related to what I'm doing now and is super useful for `datasets` in general. And then  if we agree that having more flexibility in providing parameters to the viewer is required, I can implement the second one. Let me know what you think :) 

> We can add new metadata yaml field (say, "custom_configs_info"), so that we can provide smth like:

Love it ! Some other ideas to name the "custom_configs_info" field: "configs", "parameters", "config_args", "configurations"

> it might require changes in interaction with the viewer on the hub side - to parse these configurations, as they not default configurations (not in BUILDER_CONFIGS list)

If we update the `get_dataset_config_names()` function in `datasets` in inspect.py we should be fine - that's what the viewer is using

> Overall, I would start from implementing the first solution since it's related to what I'm doing now and is super useful for datasets in general. And then if we agree that having more flexibility in providing parameters to the viewer is required, I can implement the second one. Let me know what you think :)

Actually I feel like the second solution includes the first use case you mentioned. If you implement the second solution, then users would just have to add a few lines of YAML and their directories would be considered configurations no ? Maybe there's no need to implement two different logics to do the same thing

is there any update on this? 🕵🏻

@merveenoyan I haven't started working on this yet, working on adding configs to packaged datasets instead: https://github.com/huggingface/datasets/pull/5213 because this both would allow you to solve your issue and is a frequently requested feature.

adding arbitrary parameters to yaml would be my next task i think!

@merveenoyan ignore my comment above, I'm switching to this task now :D

I want to be able to create folders in a model.

Addressed in #5331