Updates to fix the Segmentation Fault error encountered when following the steps in the README file.

Updates include:

1. Update to a multi-stage Dockerfile.
2. Set up ldconfig to load nccl-gib in the Dockerfile

Explanation:

• The use of these ldconfig lines were not correct earlier. Prior to the above multi-stage build process, since /usr/local/gib/lib64 does not exist in the Docker environment, the ldconfig has no effect since it doesn't find the libnccl / libnccl-net files that will (eventually) be there when we run the workload using a pyxis/enroot --container-mount option.
• The NeMo 24.12 container is built with ENV NCCL_VERSION=2.22.3, but nccl-gib on a3-ultra requires 2.23 or greater.
  Now that we are using the multi-stage build, we're copying a compatible libnccl into the container and configuring ldconfig properly. That way when pytorch / NeMo starts, it loads both our nccl-gib network plugin and the compatible libnccl.so.

Note: The example uses NeMo 1.0, and work to support NeMo 2.0 is in progress.

Submission Checklist

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• Test all changes with pre-commit in a local branch #
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• Update all applicable documentation
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