refactor: Remove separate moe_mesh references

[edjson]

What does this PR do ?

Removes separate moe_mesh parameter form NeMoAutoModel so users only need to pass in device_mesh to the NeMoAutoModel class.

Changelog

• Add specific line by line info of high level changes in this PR.

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Additional Information

• Follows up on fix: resolve PT 2.11 DeviceMesh deprecation warnings and unify EP mesh #1684
• closes Remove separate moe_mesh references after https://github.com/NVIDIA-NeMo/Automodel/pull/1684 #1699

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[edjson]

Hello @adil-a, I am just opening up this draft to talk more about the design changes. My current plans are to:

1. Remove the moe_mesh parameter from auto_model.py on line 468
2. Delete device_mesh.py
3. Update all the device_mesh.py imports to mesh_utils.py

Before I begin, should I initialize moe_mesh in MeshContext.__post_init__, or make moe_mesh from calling it in from_pretrained?