Nest default joblib cache dir into .dspy_cache

[amas0]

This brings the various caching functionality of dspy into one parent dir (by default) and avoids the creation of a non-hidden cachedir_joblib directory in the user's home folder.

Implements #7628.

This is the most minimal change to address the above issue.

I do believe that there might be a benefit in trying to clean up the organization of the cache directories in a more general sense. Just from doing a closer investigation, it seems that there are three areas within this library where some kind of disk caching occurs:

1. Joblib caching, which seems to be only used in the DSP submodule (after this change it will default to .dspy_cache/cachedir_joblib)
2. Finetuning, where by default a .dspy_cache/finetune directory is created
3. litellm cacheing, which is also added to .dspy_cache

Of these, 1 and 3 both initialize on import of the top level dspy module, so even if you aren't doing anything that needs the cache, you are still creating the various directories and such. You also kind of have to go hunting to find out how these caches are configured within the library as they are tucked various similarly named files, e.g. cache_utils.py / utils/caching.py, etc.

A potentially nice reorganization could do module-level configuration for the "parent" cache directory (which .dspy_cache does now, but mostly hardcoded in three different areas within the codebase), and then other components could just build off of this (such as a from dspy.cache import DSPY_CACHE_PATH or something). Something like this would make future components that require caching to continue to maintain it in a centralized location.

If that idea sounds useful at all, I'd be happy to take a closer look at implementing something (otherwise just implementing this PR resolves my original issue, so I'm happy either way 👍 )

[okhat]

Hey @amas0 ! This is amazing, and I like the longer-term discussion you wrote.

We have a number of cache improvements that are quite needed. Organizational things make sense. We also want them to be programmable, e.g. dspy.cache, and we want to switch to FanoutCache instead of DiskCache, etc.

Anyway, will likely merge this shortly.