Fix duplicate macro definition and incorrect error message

[hobostay]

Summary

This PR fixes two code quality issues in the FlashMLA codebase:

1. Remove duplicate TRAP_ONLY_DEVICE_ASSERT macro definition in csrc/utils.h

   • The macro was defined twice with identical content (lines 41-47 and 49-55)
   • While the second definition was effectively skipped due to the #ifndef guard, this is redundant code that should be cleaned up

2. Fix incorrect error message in csrc/api/dense_decode.h (line 61)

   • The error message incorrectly stated "Only head_size_v == 576 is supported"
   • The actual condition checks for head_size_v == 512
   • This caused misleading error output when the check failed

Changes

• csrc/utils.h: Removed the duplicate TRAP_ONLY_DEVICE_ASSERT macro definition
• csrc/api/dense_decode.h: Fixed error message to correctly reflect the validated value

Test plan

• These are straightforward code quality fixes with no behavioral changes
• The duplicate macro removal does not affect functionality (both definitions were identical)
• The error message fix improves user experience by providing accurate information

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