Add example compose callback and test

tests/test_apps/app_with_callbacks/app_with_log_compose_callback/__init__.py

@@ -0,0 +1 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved

tests/test_apps/app_with_callbacks/app_with_log_compose_callback/config.yaml

@@ -0,0 +1,14 @@
+age: 7
+name: James Bond
+
+hydra:
+  job:
+    name: test
+  callbacks:
+    log_compose:
+      _target_: hydra.experimental.callbacks.LogComposeCallback
+
+defaults:
+  - config_schema
+  - _self_
+  - group: a

tests/test_apps/app_with_callbacks/app_with_log_compose_callback/group/a.yaml

@@ -0,0 +1 @@
+name: a

tests/test_apps/app_with_callbacks/app_with_log_compose_callback/my_app.py

@@ -0,0 +1,32 @@
+# Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved
+from dataclasses import dataclass
+from typing import Dict
+
+from omegaconf import MISSING
+
+import hydra
+from hydra.core.config_store import ConfigStore
+from hydra.core.hydra_config import HydraConfig
+
+
+@dataclass
+class Config:
+    age: int = MISSING
+    name: str = MISSING
+    group: Dict[str, str] = MISSING
+
+
+ConfigStore.instance().store(name="config_schema", node=Config)
+ConfigStore.instance().store(name="config_schema", node=Config, group="test")
+
+
+@hydra.main(version_base=None, config_path=".", config_name="config")
+def my_app(cfg: Config) -> None:
+    print(
+        f"job_name: {HydraConfig().get().job.name}, "
+        f"name: {cfg.name}, age: {cfg.age}, group: {cfg.group['name']}"
+    )
+
+
+if __name__ == "__main__":
+    my_app()

tests/test_callbacks.py

@@ -88,6 +88,28 @@
             r"\[JOB\] on_job_start task_function: <function my_app at 0x[0-9a-fA-F]+>",
             id="on_job_start_task_function",
         ),
+        param(
+            "tests/test_apps/app_with_callbacks/app_with_log_compose_callback/my_app.py",
+            ["age=10"],
+            dedent(
+                """\
+                [HYDRA] 
+                ====
+                Composed config .*tests/test_apps/app_with_callbacks/app_with_log_compose_callback/config
+                age: 10
+                name: James Bond
+                group:
+                  name: a
+                
+                ----
+                Includes overrides \\[.*'age=10'.*\\]
+                Used defaults \\['config_schema', 'config', 'group/a'\\]
+                ====
+                job_name: test, name: James Bond, age: 10, group: a
+                """
+            ),
+            id="on_compose_callback",
+        ),
     ],
 )
 def test_app_with_callbacks(