Deprecate parallelize API
# What does this PR do?

This PR deprecates the parallelize API now that the big model API has been tested for a bit. Using `device_map="balanced"` in the call to `from_pretrained` will do the same thing as the API, and it's still possible to pass along a custom `device_map` (although they are not in the same format).
_The documentation is not available anymore as the PR was closed or merged._