Add weight for ragged gather kernel and enable fan out in bwd ragged sort

[NuojCheng]

Description

This PR fixes the usage of ragged_gather kernel under _ring_ragged_unsort_bwd. Previously, this kernel was applied in non-fan-out way, since the backward pass the unsort requires weight balance, while the ragged gather kernel did not support. This PR adds this support and enable a fan-out style of this kernel, successfully avoid materializing the full padded-shape tensor in fp32.

FIXES: b/522526813

Tests

• Performance:
  • Not use ragged buffer: 17.79 s before 17.80s after. HBM usage gets 5GB lower.
  • Use ragged buffer: 17.53 s before 17.53s after. HBM usage gets 14GB lower.
• Solve ragged buffer size memory issue: EP=8 and EP=16 successful compilation https://paste.googleplex.com/6743176042905600
• EP=8 and FSDP=64 use ragged_buffer_factor=3.0: xprof

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/kernels/ragged/ragged_gather.py	71.42%	11 Missing and 1 partial ⚠️

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