[Version] Auto generate version using setuptool_scm

[tjtanaa]

PLEASE FILL IN THE PR DESCRIPTION HERE ENSURING ALL CHECKLIST ITEMS (AT THE BOTTOM) HAVE BEEN CONSIDERED.

Purpose

This PR uses setuptool_scm to auto generate version tag based on the github release tag.

Following upstream, the vllm-omni version will also contains platform tag e.g.

For release version

vllm-omni-0.15.0rc2+cuda (on cuda)
vllm-omni-0.15.0rc2+rocm (on rocm)
vllm-omni-0.15.0rc2+npu (on npu)
vllm-omni-0.15.0rc2+xpu (on xpu)

For Dev version

vllm-omni-0.15.0rc2.dev22+gc779d43af.cuda (on cuda)
vllm-omni-0.15.0rc2.dev22+gc779d43af.rocm (on rocm)
vllm-omni-0.15.0rc2.dev22+gc779d43af.npu (on npu)
vllm-omni-0.15.0rc2.dev22+gc779d43af.xpu (on xpu)

Following vLLM, this PR also includes the feature to override the version VLLM_OMNI_VERSION_OVERRIDE. However, if developer uses VLLM_OMNI_VERSION_OVERRIDE, platform tag will not automatically be appended. E.g. VLLM_OMNI_VERSION_OVERRIDE=0.16.0 pip install -e . , the version you get is vllm-omni==0.16.0, not vllm-omni==0.16.0+cuda

Retaining the convention set in vLLM repo, SETUPTOOLS_SCM_PRETEND_VERSION is not exposed directly.

Test Plan

Tested with the following command:

VLLM_OMNI_TARGET_DEVICE=xpu python3 -m pip install -e . --no-build-isolation
VLLM_OMNI_TARGET_DEVICE=rocm python3 -m pip install -e . --no-build-isolation

python3 -m pip install -e . --no-build-isolation

# create a tag in my fork with v0.16.0, then 
python3 -m pip install -e . --no-build-isolation

Test Result

vllm-omni-0.15.0rc2.dev22+gc779d43af.rocm
vllm-omni-0.15.0rc2.dev22+gc779d43af.xpu

vllm-omni-0.15.0rc2.dev22+gc779d43af.rocm

vllm-omni-0.16.0+rocm

---

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
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft.

BEFORE SUBMITTING, PLEASE READ https://github.com/vllm-project/vllm-omni/blob/main/CONTRIBUTING.md (anything written below this line will be removed by GitHub Actions)

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