[Data] Improve warning message when read task is large

[bveeramani]

Why are these changes needed?

When a read task is large, Ray Data prints a message like this for each read task:

WARNING: the read task size (17157648427 bytes) is larger than the reported output size of the task (None bytes). This may be a size reporting bug in the datasource being read from.

The message isn't actionable, hard to understand, and spammy (it can be printed thousands of times). To address these issues, this PR makes Ray Data emit a warning like this once:

UserWarning: The serialized size of your read function named 'read_fn' is 128.0MB. This size relatively large. As a result, Ray might excessively spill objects during execution. To fix this issue, avoid accessing self or other large objects in 'read_fn'.

Related issue number

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• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bveeramani]

Changing this from a method to a property for consistency with the newly-added read_fn property