Generalize promotion script and add multi-arch keep-list filter

[HereThereBeDragons]

Renames promote_from_rc_to_final.py -> promote_packages.py to reflect its broader scope. The script now handles three orthogonal axes:

  --src-version-type   {rc, a}         (default: rc)
  --dest-version       {release, rc<N>, a<YYYYMMDD>}  (default: release)
  --keep-gfx-archs     positive arch list, e.g. gfx1201,gfx1010

Common transitions covered: rc -> release (existing default), a -> release, a -> rc. --skip-version-promotion (mutex with the version flags; requires --keep-gfx-archs) runs only the arch-trim pass on already- versioned wheels.

Multi-arch keep-list pass — for aggregator wheels (rocm, torch, torchvision, ...) and per-gfx wheels:

• Per-gfx wheels for non-kept archs are skipped (and deleted with --delete-old-on-success).
• METADATA / PKG-INFO loses Provides-Extra: device-gfx<N> and matching Requires-Dist: ...-gfx<N> lines for non-kept archs.
• requires.txt loses [device-gfx<N>] sections and [device-all] body deps for non-kept archs.
• _dist_info.py loses AVAILABLE_TARGET_FAMILIES.append('gfx<N>') for non-kept archs.
• When the package's default arch is dropped, three sites are repointed at keep[0] (so the [device] extra still resolves): DEFAULT_TARGET_FAMILY in _dist_info.py, the bare extra == "device" line in METADATA, and the [device] body in requires.txt.
• Single-arch packages are auto-detected and pass through unchanged.
• Empty intersection of keep list and found archs raises ValueError.

Other changes:

• New tests/promote_keep_list_test.py: unit tests for the keep-list helpers, version-string transitions, and argparse mutex/validation.
• tests/promote_from_rc_to_final_test.py -> promote_packages_test.py.
• third_party/change_wheel_version: skip dir / dist-info / data renames when old_slug == new_slug, so --skip-version-promotion can repack a wheel without touching its version. Same-version guard tightened to use the resolved new_version.
• upload_release_packages.py docstring updated to point at the new script name.
• how_to_do_release.md: new subsections for nightly (a) promotion (a -> release, a -> rc) and the multi-arch --keep-gfx-archs / --skip-version-promotion workflows.

[HereThereBeDragons]

for whatever reason git does not recognize the renaming of promote_from_rc_to_final.py -> promote_packages.py.
you might therefore want to checkout main branch and my promote_packages.py and just copy it into promote_from_rc_to_final.py to just see the partial changes.