Fix detectron2 import (#41510)

* fix

* fix

* typo

src/transformers/utils/import_utils.py

@@ -724,7 +724,15 @@ def is_datasets_available() -> bool:
 
 @lru_cache
 def is_detectron2_available() -> bool:
-    return _is_package_available("detectron2")
+    # We need this try/except block because otherwise after uninstalling the library, it stays available for some reason
+    # i.e. `import detectron2` and `import detectron2.modeling` still work, even though the library is uninstalled
+    # (the package exists but the objects are not reachable) - so here we explicitly try to import an object from it
+    try:
+        from detectron2.modeling import META_ARCH_REGISTRY  # noqa
+
+        return True
+    except Exception:
+        return False
 
 
 @lru_cache