[Data] [stats] Add RefBundle retrieval time metric to iterator dataset stats

[xinyuangui2]

Why These Changes Are Needed

This PR adds a new metric to track the time spent retrieving RefBundle objects during dataset iteration. This metric provides better visibility into the performance breakdown of batch iteration, specifically capturing the time spent in get_next_ref_bundle() calls within the prefetch_batches_locally function.

Related Issue Number

N/A

Example

  dataloader/train = {'producer_throughput': 8361.841782656593, 'iter_stats': {'prefetch_block-avg': inf, 'prefetch_block-min': inf, 'prefetch_block-max': 0, 'prefetch_block-total': 0, 'get_ref_bundles-avg': 0.05172277254545271, 'get_ref_bundles-min': 1.1991999997462699e-05, 'get_ref_bundles-max': 11.057470971999976, 'get_ref_bundles-total': 15.361663445999454, 'fetch_block-avg': 0.31572694455743233, 'fetch_block-min': 0.0006362799999806157, 'fetch_block-max': 2.1665870369999993, 'fetch_block-total': 93.45517558899996, 'block_to_batch-avg': 0.001048687573988573, 'block_to_batch-min': 2.10620000302697e-05, 'block_to_batch-max': 0.049948245999985375, 'block_to_batch-total': 2.048086831999683, 'format_batch-avg': 0.0001013781433686053, 'format_batch-min': 1.415700000961806e-05, 'format_batch-max': 0.009682661999988795, 'format_batch-total': 0.19799151399888615, 'collate-avg': 0.01303446213312943, 'collate-min': 0.00025646699998560507, 'collate-max': 0.9855495820000328, 'collate-total': 25.456304546001775, 'finalize-avg': 0.012211385266257683, 'finalize-min': 0.004209667999987232, 'finalize-max': 0.3785081949999949, 'finalize-total': 23.848835425001255, 'time_spent_blocked-avg': 0.04783407008137157, 'time_spent_blocked-min': 1.2316999971062614e-05, 'time_spent_blocked-max': 12.46102861700001, 'time_spent_blocked-total': 93.46777293900004, 'time_spent_training-avg': 0.015053571562211652, 'time_spent_training-min': 1.3704999958008557e-05, 'time_spent_training-max': 1.079616685000019, 'time_spent_training-total': 29.399625260999358}}

Checks

• 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 :(

[gemini-code-assist]

Code Review

This pull request introduces a new metric, iter_get_ref_bundles_s, to measure the time spent retrieving RefBundles during dataset iteration. The changes are well-integrated across the stats collection, reporting, and testing components. I've found one area for improvement to ensure the new metric is captured consistently.

[srinathk10]

LGTM