Fix gather_objects when Megatron Core is unavailable

[fallintoplace]

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What does this PR do ?

Fix gather_objects() so it does not crash when Megatron Core is unavailable and still gathers results in plain torch.distributed setups.

Collection: Common

Changelog

• avoid touching parallel_state when Megatron Core could not be imported
• preserve the existing Megatron data-parallel path when Megatron Core is available
• fall back to the default torch.distributed group when Megatron Core is unavailable
• add regression coverage for the no-Megatron/no-DDP path and the no-Megatron/plain-DDP gather path

Usage

predictions = gather_objects(predictions, main_rank=0)

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Additional Information

• Related to #
• I couldn't run pytest in this checkout because the active interpreter is missing pytest, so I verified the change with python3 -m compileall, git diff --check, and a focused isolated smoke test that exercises the no-Megatron branches.

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