[train] Default to disabling Ray Train collective util timeouts

[justinvyu]

Description

Ray Train's framework agnostic collective utilities (ray.train.collective.barrier, ray.train.collective.broadcast_from_rank_zero) currently timeout after 30 minutes if not all ranks join the operation. ray.train.report uses these collective utilities internally, so users who don't call report on every rank can run into deadlocks. For example, the report barrier can deadlock with another worker waiting on others to join a backward pass collective.

This PR changes the default Ray Train collective behavior to never timeout and to only log warning messages about the missing ranks. User code typically already has timeouts such as NCCL timeouts (also 30 minutes by default), so the extra timeout here doesn't really help and increases the user burden of keeping track of environment variables to set when debugging hanging jobs.

New default: RAY_TRAIN_COLLECTIVE_TIMEOUT_S=-1

This PR also generalizes the environment variable name: RAY_TRAIN_REPORT_BARRIER -> RAY_TRAIN_COLLECTIVE.

[gemini-code-assist]

Code Review

This pull request correctly implements the plan to disable Ray Train collective utility timeouts by default, which should improve user experience by reducing deadlocks and configuration overhead. The renaming of environment variables from REPORT_BARRIER to a more general COLLECTIVE is a good improvement. The logic changes and the addition of a new test case to verify the behavior with no timeout are solid. I've added a couple of suggestions: one to fix a formatting issue in an error message and another to make a new test more explicit and robust.