use partitioner instance directly in to_backend (#2513)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2513

to_backend either takes partitioner or a dict of partitioner `key: method_name, value: partitioner`. We shouldn't do key as the backend name and value as the partitioner.

Reviewed By: mergennachin

Differential Revision: D55078939

fbshipit-source-id: 7c2b8587095d2227d17a198c3b4e975910f4df00