Fix gangs coordinator rank check

[joaopedroassad]

What does this PR do? Please describe:

Gangs.__post_init__ enforces that when root.rank == 0, the coordinator process must be rank 0 in every parallel gang. The check read:

if self.dp.rank != 0 or self.tp.rank != 0 and self.pp.rank != 0:

Since Python binds and tighter than or, this evaluates as dp.rank != 0 or (tp.rank != 0 and pp.rank != 0). As a result, a root.rank == 0 process that is non-zero in exactly one of the tensor- or pipeline-parallel gangs (for example tp.rank == 1, pp.rank == 0, or tp.rank == 0, pp.rank == 1) passes validation, contradicting the error message ("must be rank 0 in all parallel gangs"). This PR joins all three checks with or, adds parametrized regression tests, and adds a CHANGELOG entry.

No associated issue.

Does your PR introduce any breaking changes? If yes, please list them:

No. It only makes the validation reject coordinator configurations it was always meant to reject; valid configurations (rank 0 in every parallel gang) behave exactly as before.

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? No prior issue; this is a one-line logic-bug fix accompanied by a test.
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? No documentation change is needed for this fix.
• Did you write any new necessary tests? Added parametrized tests in tests/unit/test_gang.py covering the previously-accepted (tp=1, pp=0) and (tp=0, pp=1) coordinator configurations.
• Did you verify new and existing tests pass locally with your changes? Verified the fix with a standalone reproduction of the check and ran isort / black / flake8 clean. The full pytest suite could not run locally because the native fairseq2n module has no macOS build; the new tests exercise the real Gangs class and run in CI.
• Did you update the CHANGELOG?

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