I didn't think that was an option at all in Safari. Are you seeing it somewhere in the Safari preferences?

The only other way you could approach it would be to target the ~/Library/Safari folder and run a script to clear caches from there.

If you enable the Develop menu in Safari (see the Advanced tab in Safari preferences) there is a Disable Caches option there. Unfortunately it does not appear to be a persistent setting, so quitting and re-running Safari re-enables caching.