Title: Fix setup_env_ranks to Properly Set Environment Variables Instead of Raising Error

[fabiosanger]

This pull request addresses an issue in setup_env_ranks where, under certain conditions, the function raises an error instead of setting the necessary MPI-related environment variables (LOCAL_RANK, RANK, and WORLD_SIZE). The intended behavior is to properly map Open MPI variables (OMPI_COMM_WORLD_*) to the variables expected by DeepSpeed/PyTorch, but the code currently raises an EnvironmentError if these Open MPI variables are not found.

With this fix, setup_env_ranks will:

• Correctly detect and map the required Open MPI environment variables.
• Only raise an error if there is genuinely no valid way to obtain rank information from the environment (e.g., both Open MPI variables and any fallback mechanism are unavailable).

Fix #6895

[fabiosanger]

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