Feature Request: adopt ant's AMem as an optional API to offload NCCL memory in RL scenario

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when

https://github.com/inclusionAI/asystem-amem Amen is an open source NCCL plugin for fast offload/reload, which could potential save up to 10GB memory saving for rollout.

**Describe the solution you'd like**
Add an optional dependency with Amem to build communication groups.

The issue does not name files or tests. Start by locating how Megatron-LM builds communication groups and declares optional dependencies, then determine where an AMem-backed path would fit for RL rollout memory offload. Done means the optional dependency can be enabled without affecting existing users and the communication groups use it when configured.

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