add weight_sum_fp32 config

[suexu1025]

Description

Add config flag weight_sum_fp32 for whether to use full fp32 precision for weight_sum during final unpermute in moe

Tests

final eval loss at 300 steps
2.394 (cloudlog)(https://cloudlogging.app.goo.gl/Q5o2tac9aypGGMyV6)
2.393 (cloudlog)(https://cloudlogging.app.goo.gl/L2N43dAZiHap1Djk7)

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.