Remove dependency on megatron.training within megatron.core

[ananthsub]

What does this PR do ?

Re-attempt #2113 with fixes.
#2113 was reverted because unwrap_model unwrapped the FullyShardedDataParallel module, which meant this code no longer had a reference to the TransformerConfig to look up the expert information from.

⚠️ For major changes (either in lines of code or in its impact), please make sure to first share discuss a design-doc with the team.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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Pre-checks

• I want this PR in a versioned release and have added the appropriate Milestone (e.g., Core 0.8)
• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

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Final Review might get declined if these requirements are not fulfilled.

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If this PR also needs to be merged into core_r* release branches, after this PR has been merged, select Cherry-pick to open a new PR into the release branch.

For MRs into `dev` branch

The proposed review process for `dev` branch is under active discussion.

MRs are mergable after one approval by either [email protected] or [email protected].

Merging your PR

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[ananthsub]

/ok to test d87f1c7

[ananthsub]

/ok to test d87f1c7