Don't duplicate data when encoding audio or image
Right now if you pass both the `bytes` and a local `path` for audio or image data, then the `bytes` are unnecessarily written in the Arrow file, while we could just keep the local `path`.

This PR discards the `bytes` when the audio or image file exists locally.

In particular it's common for audio datasets builders to provide both the bytes and the local path in order to work for both streaming (using the bytes) and non-streaming mode (using a local file - which is often required for audio).

cc @patrickvonplaten 