Hard-coded RankGenerator arguments in megatron.core.parallel_state.create_all_gather_groups (used for independent AG).

**Describe the bug**

https://github.com/NVIDIA/Megatron-LM/pull/3249 was not finished with review before it was merged, which had a Claude comment that was unaddressed: https://github.com/NVIDIA/Megatron-LM/pull/3249#discussion_r2989277554

**Steps/Code to reproduce bug**

- Potential bug but mainly code quality.

**Expected behavior**

- Ranks should be generated based on the current PG collection rank assignments.

**Additional context**

Add any other context about the problem here.