Restore GROUPED_CONV_HEURISTIC_REPORT.md dropped by mirror sync

This report was added to ROCm/rocm-libraries by #7761 (commit 237b766,
"[CK][CK TILE] Clean up tile_engine grouped_conv harness", merged
2026-05-29) and force-tracked there despite the dispatcher/heuristics
*.md ignore rule. The pr-merge-sync-patches workflow stages with a plain
`git add .`, which honors the mirror's .gitignore and silently skips
force-tracked files, so this file never landed in the mirror while its
four sibling docs did.

Restoring by hand (git add -f) for parity; content is byte-identical to
the monorepo develop copy.