text: Title: Webdataset data format problem
  Body: ### Describe the bug
  
  Please see https://huggingface.co/datasets/ejschwartz/idioms/discussions/1
  
  Error code: FileFormatMismatchBetweenSplitsError
  
  All three splits, train, test, and validation, use webdataset. But only the train split has more than one file. How can I force the other two splits to also be interpreted as being the webdataset format?  (I don't think there is currently a way, but happy to be told that I am wrong.)
  
  ### Steps to reproduce the bug
  
  ```
  import datasets
  datasets.load_dataset("ejschwartz/idioms")
  
  ### Expected behavior
  
  The dataset loads.  Alternatively, there is a YAML syntax for manually specifying the format.
  
  ### Environment info
  
  - `datasets` version: 3.2.0
  - Platform: Linux-6.8.0-52-generic-x86_64-with-glibc2.35
  - Python version: 3.10.12
  - `huggingface_hub` version: 0.28.1
  - PyArrow version: 19.0.0
  - Pandas version: 2.2.3
  - `fsspec` version: 2024.9.0