Remove unstable gather-rounds checkpoint exchange

[deepujain]

• I, the PR author, have personally reviewed every line of this PR.

What does this PR do?

The gather_rounds checkpoint exchange path has caused numerical instability in production workloads. This change removes that implementation and configuration choice while retaining the broadcast default and the gather_object debugging option.

The load-strategy documentation and FP8 test parameterization are updated so users can no longer select the unstable path. Unknown exchange algorithms continue to fail explicitly with NotImplementedError.

Issue tracking

Fixes #3738

Contribution process

Pre-checks

• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

Validation

• A focused CPU check previously verified that exchange_by_distribution(..., exchange_algo="gather_rounds") now raises NotImplementedError; the repaired diff preserves that removal unchanged.
• Repository-wide search found no remaining gather_rounds references.
• python3 -m py_compile and git diff --check passed for all changed files.
• Independent exact-head review found no blocking correctness or compatibility defects.

The repaired commit is rebased on current main and signed. The distributed FP8 checkpoint test requires the repository's multi-GPU environment and remains a hosted CI gate.

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