```python
Return a folder path to cache arbitrary files.

`huggingface_hub` provides a canonical folder path to store assets. This is the
recommended way to integrate cache in a downstream library as it will benefit from
the builtins tools to scan and delete the cache properly.

The distinction is made between files cached from the Hub and assets. Files from the
Hub are cached in a git-aware manner and entirely managed by `huggingface_hub`. See
[related documentation](https://huggingface.co/docs/huggingface_hub/how-to-cache).
All other files that a downstream library caches are considered to be "assets"
(files downloaded from external sources, extracted from a .tar archive, preprocessed
for training,...).

Once the folder path is generated, it is guaranteed to exist and to be a directory.
The path is based on 3 levels of depth: the library name, a namespace and a
subfolder. Those 3 levels grants flexibility while allowing `huggingface_hub` to
expect folders when scanning/deleting parts of the assets cache. Within a library,
it is expected that all namespaces share the same subset of subfolder names but this
is not a mandatory rule. The downstream library has then full control on which file
structure to adopt within its cache. Namespace and subfolder are optional (would
default to a `"default/"` subfolder) but library name is mandatory as we want every
downstream library to manage its own cache.

Expected tree:
```text
assets/
└── datasets/
│   ├── SQuAD/
│   │   ├── downloaded/
│   │   ├── extracted/
│   │   └── processed/
│   ├── Helsinki-NLP--tatoeba_mt/
│       ├── downloaded/
│       ├── extracted/
│       └── processed/
└── transformers/
├── default/
│   ├── something/
├── bert-base-cased/
│   ├── default/
│   └── training/
hub/
└── models--julien-c--EsperBERTo-small/
├── blobs/
│   ├── (...)
│   ├── (...)
├── refs/
│   └── (...)
└── [ 128]  snapshots/
├── 2439f60ef33a0d46d85da5001d52aeda5b00ce9f/
│   ├── (...)
└── bbc77c8132af1cc5cf678da3f1ddf2de43606d48/
└── (...)
```


Args:
library_name (`str`):
Name of the library that will manage the cache folder. Example: `"dataset"`.
namespace (`str`, *optional*, defaults to "default"):
Namespace to which the data belongs. Example: `"SQuAD"`.
subfolder (`str`, *optional*, defaults to "default"):
Subfolder in which the data will be stored. Example: `extracted`.
assets_dir (`str`, `Path`, *optional*):
Path to the folder where assets are cached. This must not be the same folder
where Hub files are cached. Defaults to `HF_HOME / "assets"` if not provided.
Can also be set with `HF_ASSETS_CACHE` environment variable.

Returns:
Path to the cache folder (`Path`).

Example:
```py
>>> from huggingface_hub import cached_assets_path  
>>> cached_assets_path(library_name="datasets", namespace="SQuAD", subfolder="download")
PosixPath('/home/wauplin/.cache/huggingface/extra/datasets/SQuAD/download')  
>>> cached_assets_path(library_name="datasets", namespace="SQuAD", subfolder="extracted")
PosixPath('/home/wauplin/.cache/huggingface/extra/datasets/SQuAD/extracted')  
>>> cached_assets_path(library_name="datasets", namespace="Helsinki-NLP/tatoeba_mt")
PosixPath('/home/wauplin/.cache/huggingface/extra/datasets/Helsinki-NLP--tatoeba_mt/default')  
>>> cached_assets_path(library_name="datasets", assets_dir="/tmp/tmp123456")
PosixPath('/tmp/tmp123456/datasets/default/default')
```
```