TL/UCP: Relax Allreduce Ring Constraints

[wfaderhold21]

What

Addresses #1293

[greptile-apps]

Greptile Summary

The PR allows UCP ring allreduce to process counts that are not divisible by the team size and selects ring by default for allreduce messages of at least 4 MiB.

• Uses remainder-aware block sizes and offsets in ring allgather communication and initial source copying.
• Removes the allreduce ring divisibility restriction.
• Adds non-divisible-count and AVG fallback coverage across supported host and GPU memory types.
• Restores test-process environment variables to their original set or unset state after context creation.

Confidence Score: 5/5

The PR appears safe to merge, with no concrete changed-code failure identified.

The uneven ring paths consistently calculate matching block sizes and offsets, score selection retains a working generic fallback for unsupported ring operations, and the test harness restores environment state after its configuration consumers run.

Important Files Changed

Filename	Overview
src/components/tl/ucp/allgather/allgather_ring.c	Replaces equal-block arithmetic with consistent remainder-aware counts and offsets for sends, receives, and the initial copy.
src/components/tl/ucp/allreduce/allreduce.h	Changes default allreduce selection to knomial below 4 KiB, SRA knomial through 4 MiB, and ring for larger messages.
src/components/tl/ucp/allreduce/allreduce_ring.c	Removes the divisibility guard so the composed reduce-scatter and allgather ring can use uneven block partitioning.
test/gtest/coll/test_allreduce.cc	Adds broad correctness coverage for non-divisible ring counts and AVG fallback under forced and default selection.
test/gtest/common/test_ucc.cc	Preserves each unique environment variable's original presence and value, including unsetting variables that were initially absent.

_{Reviews (1): Last reviewed commit: "TEST: add allreduce pre-op fall back tes..." | Re-trigger Greptile}