Every current release of the kernels package (0.13 through 0.16) builds its
dependency tables with huggingface_hub >= 1.0's strict-dataclass API, and with
an older hub the breakage is not contained to the requested backend: import
kernels raises at module scope, and diffusers imports kernels whenever it is
installed, so a single on-demand install (an explicit flash3/flash4 attention
request on a stack pinned to hub < 1.0) permanently breaks every later
diffusers pipeline import on the box until the package is removed. Reproduced
against hub 0.36.2 with kernels 0.13.0 and 0.16.0: the HunyuanVideo-1.5
pipeline import dies in hub's strict-dataclass validator both times.
_ensure_attention_backend_installed now checks the resident hub version before
installing kernels (_kernels_hub_compatible) and refuses on < 1.0, logging why
and falling back to the native default, which is the best-effort contract the
installer already promises for an uninstallable wheel. The refusal is a policy
decision, not a failed attempt, so it is not memoised and a later request on a
fixed environment can still install. An undeterminable hub version keeps the
previous permissive behaviour, and the gate applies only to the kernels
package: sage/flash-attn/xformers wheels do not import hub at module scope.
Tests: the refusal (nothing memoised), the hub >= 1.0 allow, the
package-scoping, and the version-parse fallback.
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