fix expert_parallelism_name assignment

[khatwanimohit]

Description

_expert_parallelism_name was unconditionally set to "attn_dp_expert" whenever attention == "vllm_rpa", even when enable_dp_attention is false. Added the enable_dp_attention check so it falls back to "expert" when DP attention is not enabled.

Thank you Nico for pointing this out!

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FIXES: b/123456
FIXES: #123456

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/layers/moe.py	0.00%	0 Missing and 1 partial ⚠️

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[NicoGrande]

LGTM!