build: add dynamic git-versioning and drop rc0 pre-release tag

[ko3n1g]

Summary

• megatron/core/package_info.py: drop rc0 pre-release tag (PRE_RELEASE = '') and append +<short-sha> to __version__ at import time using only the git binary
• .github/workflows/_build_test_publish_wheel.yml: set NO_VCS_VERSION=1 in both the step env (for the host python call that computes EXPECTED_RELEASE_NUMBER) and in the docker run -e flag (for the manylinux build container where git is unavailable)

The opt-out flag NO_VCS_VERSION=1 keeps wheel version strings clean (e.g. 0.18.0 instead of 0.18.0+abc1234) while still allowing local installs to carry the SHA for traceability.

Example

>>> import megatron.core; megatron.core.__version__
'0.18.0+f63874e'

# Opt-out:
NO_VCS_VERSION=1 python -c "import megatron.core; print(megatron.core.__version__)"
0.18.0

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[ko3n1g]

/ok to test

[ko3n1g]

/ok to test