fix: actually set KERAS_BACKEND to chosen backend

Add warning if KERAS_BACKEND and actually loaded backend do not match.
This can happen if keras is imported before BayesFlow.

Author: vpratz

bayesflow/__init__.py

@@ -49,6 +49,7 @@ def __init__(self, display_name, package_name, env_name, install_url, priority):
 
             found_backends.sort(key=lambda b: b.priority)
             chosen_backend = found_backends[0]
+            os.environ["KERAS_BACKEND"] = chosen_backend.env_name
 
             warnings.warn(
                 f"Multiple Keras-compatible backends detected ({', '.join(b.display_name for b in found_backends)}).\n"
@@ -71,6 +72,9 @@ def __init__(self, display_name, package_name, env_name, install_url, priority):
 
     from bayesflow.utils import logging
 
+    if keras.backend.backend().lower() != os.environ["KERAS_BACKEND"].lower():
+        logging.warning("Automatic backend selection failed, most likely because Keras was imported before BayesFlow.")
+
     logging.info(f"Using backend {keras.backend.backend()!r}")
 
     if keras.backend.backend() == "torch":