[Compilation fix] add stubs to allow compilation without sm100

[mickaelseznec]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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Purpose

Previously, when setting TORCH_CUDA_ARCH_LIST="9.0" for example, vllm would complain because some symbols were missing.

sm100_cutlass_mla_get_workspace_size is indeed referenced in csrc/torch_bindings.cpp

Add stubs to fix this situation.

Test Plan

Test Result

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request fixes a compilation failure when vLLM is built without support for sm100 architectures by adding stub implementations for missing symbols. To fully resolve the issue, I've added a critical comment to include the missing stub for sm100_cutlass_mla_decode.

[mickaelseznec]

@LucasWilkinson for viz since you reviewed #16032

(I don't know why the reviewers haven't been set with my PR)

[LucasWilkinson]

Sorry for the delay! is this still needed after: #21020 ?

[mickaelseznec]

np! Yes I confirm that you fixed it there already :) Closing