[Community][Main] feat(moe): Adding context parallel support to eager attention implementation

[yuzhongw-nvidia]

What does this PR do ?

Sometimes, certain attention and mask implementations are difficult to write a fused / optimized implementation in a short period of time. However, we still need to run experiments to verify their effectiveness.
At such times, we need to fallback to the eager mode. Therefore, I added a switch to fallback to the eager implementation of attention:

--fallback-to-eager-attn

Additionally, since Megatron Core's eager attention does not support context parallelism, I provided a distributed attention implementation similar to that described in the Llama 3 paper.

PR to dev

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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
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