delete autocast, an unnecessary thorn in my side, manage dtypes directly

tests/test_attention_fallback.py

@@ -21,8 +21,9 @@ from nanochat.engine import KVCache
 
 
 def set_impl(impl):
-    """Set the implementation override ('fa3', 'sdpa', or None for auto)."""
+    """Set the implementation override ('fa3', 'sdpa', or None for auto) and re-resolve USE_FA3."""
     fa_module._override_impl = impl
+    fa_module.USE_FA3 = fa_module._resolve_use_fa3()
 
 
 def run_both_impls(fn):
@@ -343,19 +344,19 @@ class TestOverrideMechanism:
     def test_override_fa3(self):
         """Test that override='fa3' uses FA3."""
         set_impl('fa3')
-        assert fa_module._use_fa3() == True
+        assert fa_module.USE_FA3 == True
         set_impl(None)
 
     def test_override_sdpa(self):
         """Test that override='sdpa' uses SDPA."""
         set_impl('sdpa')
-        assert fa_module._use_fa3() == False
+        assert fa_module.USE_FA3 == False
         set_impl(None)
 
     def test_override_auto(self):
         """Test that override=None uses auto-detection."""
         set_impl(None)
-        assert fa_module._use_fa3() == HAS_FA3
+        assert fa_module.USE_FA3 == HAS_FA3
 
 
 if __name__ == "__main__":