[Unity][CODEGEN] Fix metal codegen when with only single working dim

[tqchen]

This PR fixes the metal introduces by a previous commit that removes the workdim remapping that caused issues for kernels with only threadIdx.x and blockIdx.x

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• cc @quic-sanirudh _{See #10317 for details}

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[tqchen]

cc @Lunderberg was caused by recent merging that bought in #14495 . This is a hotfix, will also need pick to main

[Lunderberg]

Thank you for finding the fix, and that makes sense with the different naming based on the number of block/thread dimensions. Do we have any tests of the metal codegen in CI? I was hoping to add a test to catch this failure, but it looks like Metal doesn't currently run on the CI due to being OSX-only.

[tqchen]

right i think it should be covered by tests already but was not run due to metal being osx only