[Main] Fix EP Overlap Bugs for Full-Iter CG

[Wohox]

What does this PR do ?

Fix following bugs for full-iter cg:

• Missing stream context for backward dw.
• Memory leak when using HybridEP+FP8.
• Missing record stream for HybridEP's tokens_per_expert.
• Accidentally release the input memory of grouped gemm.
  PR for dev: [Dev] Fix EP Overlap Bugs for Full-Iter CG #3163

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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• I have added relevant unit tests
• I have added relevant functional tests
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[copy-pr-bot]

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[Phlip79]

/ok to test 4260761

[Wohox]

/ok to test a40857f

[Wohox]

/ok to test 07a80b6

[Wohox]

/ok to test 92824b5

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21934133498

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21934478751

[ko3n1g]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/21935218560