[Data] Make memory an explicit map parameter

[bveeramani]

Why are these changes needed?

Configuring per-task memory is one of the easiest ways to workaround OOMs. However, most people don't know you can do this, and the current syntax is implicit:

Before

ds.map_batches(..., ray_remote_args={"memory": ...})

After

ds.map_batches(..., memory=...)

Related issue number

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[richardliaw]

This is great. Can we add some information in the docs about how to use this parameter effectively? Like a small user guide paragraph would be great.