[Bugfix] Fix hang with DP+EP on B200

[alexm-redhat]

This PR fixes the hang with DP+EP on DSR1 for B200 (TODO add more details)

[gemini-code-assist]

Code Review

This pull request addresses a hang issue on B200 GPUs when using Data Parallelism and Expert Parallelism by increasing the number of tokens in a dummy batch run from 1 to 16. While this change likely resolves the immediate problem, the use of the magic number 16 introduces a maintainability concern. I've suggested replacing it with a named constant and adding a comment to explain its purpose, which will make the code clearer for future developers.

[gemini-code-assist]

While changing 1 to 16 likely fixes the hang on B200, the value 16 is a magic number. To improve code clarity and maintainability, it would be better to define this as a named constant with a comment explaining why this specific value is necessary for the fix.

For example, you could define a constant at the top of the file:

# This value is chosen to ensure sufficient workload on all ranks to avoid
# hangs with Data Parallelism and Expert Parallelism on B200 hardware.
# See the associated pull request for more details.
_DUMMY_BATCH_TOKENS_FOR_B200_FIX = 16

And then use it here:

self.model_runner._dummy_run(_DUMMY_BATCH_TOKENS_FOR_B200_FIX, uniform_decode=True)

This would make the code's intent much clearer to future maintainers. Also, please consider updating the PR description with the details as mentioned in the TODO.