[Data] Store Ray Data logs in special subdirectory

[bveeramani]

Why are these changes needed?

Currently, Ray Data stores logs in the {session_directory}/logs directory along with all other Ray logs. This PR changes the behavior so Ray Data stores log in a dedicated {session_directory}/logs/ray-data directory.

Related issue number

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    method in Tune, I've added it in doc/source/tune/api/ under the
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  • This PR is not tested :(

[bveeramani]

cc @brycehuang30

[scottjlee]

can we also update ray data documentation accordingly with the new subdirectory? thanks!

[c21]

Let's also update all examples documentation with:

Starting execution of Dataset. Full log is in .../logs/ray-data.log