fix(runtime): autosave engine-implicit RuntimeCache via atexit + weakref

[tp5uiuc]

Summary

• Engine-implicit RuntimeCache handles (those constructed by an engine
  when RuntimeSettings(runtime_cache="/path") is used) previously relied
  solely on __del__ to autosave the kernel cache. When the engine
  survived until interpreter exit (typical for inference servers),
  __del__ fired during shutdown — by which point Python had torn
  down sys.meta_path, breaking the torchbind attribute access in
  self._handle.path and the lazy filelock import inside save().
  The resulting ImportError escaped __del__ and printed
  Exception ignored in: <function RuntimeCache.__del__> once per
  surviving handle (🐛 [Bug] Serialization fails in Runtime Cache #4359).
• Move the engine-implicit autosave anchor to atexit, which fires
  before module teardown. __del__ remains as the mid-program GC path.

Design notes

• atexit.register(partial(_autosave_at_exit, weakref.ref(self))) keeps
  the registration non-owning: a handle that dies mid-program is still
  collected normally; the atexit hook later sees a dead weakref and
  no-ops. A bound method would have defeated the weakref, hence the
  module-level free function + partial.
• The autosave logic lives once on the class as _autosave_if_enabled
  (called from both __del__ and the atexit hook). It flips
  autosave_on_del off before saving so whichever path runs first wins
  and the other no-ops — no double-save, no double-leak risk.
• __del__ calls atexit.unregister(self._atexit_token) so the
  registry size stays O(live handles) rather than O(all handles ever created) — matters for long-running processes that churn engines.
• self._atexit_token = None is the first statement in __init__, so
  __del__ can always read it safely even if a later line of __init__
  raises (partial-init defense).

Refs #4359 — the topology crash that surfaced this issue is fixed in a
separate PR; this PR addresses the autosave reliability + shutdown
noise.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Test plan

Added under TestRuntimeCacheAutosave in
tests/py/dynamo/runtime/test_000_runtime_cache.py:

• test_del_swallows_shutdown_import_error_on_path — monkey-patches
  _handle.path to raise the shutdown ImportError; asserts via
  sys.unraisablehook that nothing leaks.
• test_atexit_hook_saves_via_weakref — exercises the helper
  directly; verifies it routes through _autosave_if_enabled, saves,
  and flips autosave_on_del off.
• test_atexit_hook_no_op_on_dead_weakref — dead weakref → no-op,
  no exception.
• test_atexit_token_unregistered_after_del — atexit.unregister
  is spied to confirm __del__ cleaned up the registration with the
  specific partial token.
• Full test_000_runtime_cache.py passes locally on cpp-rt (17
  passed, 6 unrelated skips on the py-runtime whitebox tests).
• pre-commit run clean on touched files.