[feature request]: Recursive loading of image folder

Could you add an option for recursive loading of image files, with any number of layers? So you can sort more then just having multiple folders?

So rather then
```
1_Character 1
1_Character 2
1_Character 3
1_Character 4
1_Character 5
```

You can have
```
Series 1
Character 1
1_Outfit 1
1_Outfit 2
1_Character 2
Series 2
1_Character 1
Character 2
1_Outfit 1
1_Outfit 2
1_Character 3
```
Currently what i'm doing is prefixing the series or another tag to the beginning of the folder name, but it gets very cluttery with thousands of folders, yes im trying to train a Checkpoint.
```
1_Series 1 Character 1 Outfit 1
1_Series 1 Character 1 Outfit 2
1_Series 1 Character 2
1_Series 2 Character 1
1_Series 2 Character 2 Outfit 1
1_Series 2 Character 2 Outfit 2
1_Series 2 Character 3
```

But it has gotten so cluttery i am now thinking of using SymLinks as a temporary solution. So i can have one structured folder and then the unstructured training folder with SymLinks.

The issue names no files or tests. Start by locating the image-folder loading entry point used for checkpoint training and trace how folder names become training data; verify the behavior with nested folders of arbitrary depth and confirm existing flat-folder loading still works.

Written by the indexing model from the issue text.