[ray] disable capturing child tasks in placement group.

[richardliaw]

Checklist before submitting

• Did you read the contributor guide?
• Did you update the docs?
• Did you write any tests to validate this change?
• Did you update the CHANGELOG, if this change affects users?

Description

This fixes an existing bug with using a Ray-based shuffling dataloader, which we are developing separately.

Review process to land

1. All tests and other checks must succeed.
2. At least one member of the technical steering committee must review and approve.
3. If any member of the technical steering committee requests changes, they must be addressed.

[richardliaw]

cc @rkooo567 FYI

[rkooo567]

@richardliaw you said it didn't work right?

[github-actions]

Unit Test Results

     763 files  ±0       763 suites  ±0   5h 55m 48s ⏱️ ±0s
     597 tests ±0       560 ✔️ ±0       37 💤 ±0  0 ❌ ±0 
15 848 runs  ±0  11 959 ✔️ ±0  3 889 💤 ±0  0 ❌ ±0 

Results for commit 5b2f1f0. ± Comparison against base commit 5b2f1f0.

♻️ This comment has been updated with latest results.

[chongxiaoc]

LGTM