[MRV2] Make FP32 Gumbel sampling more accurate

[WoosukKwon]

The current FP32 Gumbel sampling kernel is not accurate enough. This PR improves the accuracy with a simple trick: use log1p(-u) instead of log(u), enabling higher resolution in determining the argmax winner.

[mergify]

Hi @WoosukKwon, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.