Remove calculation of padding token in moe routing loss

[HaochenYuan]

What does this PR do ?

Related issue: #1984
This PR removes the calculation of padding token in aux loss.
See detailed description in corresponding dev branch PR #2754

⚠️ 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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• 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
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[HaochenYuan]

/ok to test ccee48d

[Victarry]

LGTM. Already reviewed the PR to the dev branch.

[HaochenYuan]

/ok to test 767638c

[duncanriach]

LGTM

[Phlip79]

/ok to test 2525c62

[Phlip79]

Accidentally closed this PR, sorry about that.

[Phlip79]

@HaochenYuan can you please take a look at failing test?

[HaochenYuan]

@HaochenYuan can you please take a look at failing test?

I'm working on it. There seems to do something wrong with the ci machine.

9-task-1-0/0 [default0]:[2026-01-22 09:20:41] mcore-ci-test-1769073559-task-1-0:273:1483 [0] misc/shmutils.cc:87 NCCL WARN Error: failed to extend /dev/shm/nccl-SAQVeM to 34210180 bytes, error: No space left on device (28)
9-task-1-0/0 [default0]:
9-task-1-0/0 [default0]:[2026-01-22 09:20:41] mcore-ci-test-1769073559-task-1-0:273:1483 [0] misc/shmutils.cc:131 NCCL WARN Error while creating shared memory segment /dev/shm/nccl-SAQVeM (size 34210176), error: No space left on device (28)
9-task-1-0/0 [default0]:mcore-ci-test-1769073559-task-1-0:273:1483 [0] NCCL INFO proxy.cc:1374 -> 2
9-task-1-0/0 [default0]:mcore-ci-test-1769073559-task-1-0:273:1483 [0] NCCL INFO proxy.cc:1440 -> 2
9-task-1-0/0 [default0]:
9-task-1-0/0 [default0]:[2026-01-22 09:20:41] mcore-ci-test-1769073559-task-1-0:273:1483 [0] misc/shmutils.cc:87 NCCL WARN Error: failed to extend /dev/shm/nccl-LVYUle to 34210180 bytes, error: No space left on device (28)
9-task-1-0/0 [default0]:
9-task-1-0/0 [default0]:[2026-01-22 09:20:41] mcore-ci-test-1769073559-task-1-0:273:1483 [0] misc/shmutils.cc:131 NCCL WARN Error while creating shared memory segment /dev/shm/nccl-LVYUle (size 34210176), error: No space left on device (28)

[HaochenYuan]

@HaochenYuan can you please take a look at failing test?

Done