Fixes for gpt-oss

[cuichenx]

What does this PR do ?

Various minor fixes for gpt-oss model

1. Fix yarn mscale to match with HF implementation and disable rope fusion when mscale is used.
2. Initialize softmax offset on GPU
3. Remove lru cache for attention mask to prevent OOM with long sequence lengths
   Works with Add gpt-oss finetune recipes + fixes NVIDIA-NeMo/Megatron-Bridge#1155

⚠️ 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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For MRs into `dev` branch

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[cuichenx]

/ok to test c5b1e1e

[cuichenx]

/ok to test eedb6bb

[kvareddy]

@santhnm2, can you please take a look at this MR?