chore: prepare changelog for 2.6.4 release (#21729)
- Add [2.6.4] release section with all changes from the skipped 2.6.2/2.6.3
- Note supply chain security compromise (see #21691) as reason for skipped versions
- Includes: Neptune logger removal, LitLogger bump, val_check_interval fix,
pkg-resources fix, FSDP mixed precision fix, device_mesh type hint fix,
RichModelSummary fix, SimpleProfiler math.fsum precision fix
