[BUG] inference.py expects labeled subfolders in the --data-dir folder

**Describe the bug**
When running `inference.py --data-dir data --class-map class_map.txt --model efficientnet_b7 --num-classes 8 --checkpoint output/model_best.pth.tar` where `data` is a directory that contains images and no subfolders, the script crashes with the following output:
```
Traceback (most recent call last):
File "/home/josua/redacted/pytorch-image-models/inference.py", line 369, in
main()
File "/home/josua/redacted/pytorch-image-models/inference.py", line 224, in main
dataset = create_dataset(
^^^^^^^^^^^^^^^
File "/home/josua/redacted/pytorch-image-models/timm/data/dataset_factory.py", line 216, in create_dataset
ds = ImageDataset(
^^^^^^^^^^^^^
File "/home/josua/redacted/pytorch-image-models/timm/data/dataset.py", line 36, in __init__
reader = create_reader(
^^^^^^^^^^^^^^
File "/home/josua/redacted/pytorch-image-models/timm/data/readers/reader_factory.py", line 44, in create_reader
reader = ReaderImageFolder(root, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/josua/redacted/pytorch-image-models/timm/data/readers/reader_image_folder.py", line 82, in __init__
raise RuntimeError(
RuntimeError: Found 0 images in subfolders of /home/josua/redacted/data/unlabeled. Supported image extensions are .png, .jpg, .jpeg
```

**Expected behavior**
Instead of raising an error, I expect it to perform the inference on the (recursive) contents of `data` irrespective of the existence of labeled subfolders because labels are not necessary for inference.

**Desktop (please complete the following information):**
- OS: x86_64 Linux 6.11.5-arch1-1
- This repository version: d4dde48d
- PyTorch version w/ CUDA/cuDNN: python-pytorch-cuda 2.3.1-8, cuda 12.6.2-2

**Additional context**
Workaround: It works when, instead of placing the images in `data/` directly, one places them in `data/random_label/` where `random_label` is a random label, i.e. a random line from `class_map.txt`