Add ddp_find_unused_parameters_true alias in Fabric's DDPStrategy (#20125)

Author: 01AbhiSingh

src/lightning/fabric/strategies/ddp.py

@@ -200,6 +200,13 @@ def register_strategies(cls, strategy_registry: _StrategyRegistry) -> None:
                 description=f"DDP strategy with `start_method={start_method!r}`",
                 start_method=start_method,
             )
+        strategy_registry.register(
+            "ddp_find_unused_parameters_true",
+            cls,
+            description="Alias for `find_unused_parameters_true` and `start_method='popen'`",
+            find_unused_parameters=True,
+            start_method="popen",
+        )
 
     def _setup_distributed(self) -> None:
         self._set_world_ranks()

tests/tests_fabric/strategies/test_registry.py

@@ -42,6 +42,7 @@ def __init__(self, param1, param2):
 def test_available_strategies_in_registry():
     expected = {
         "ddp",
+        "ddp_find_unused_parameters_true",
         "deepspeed",
         "deepspeed_stage_1",
         "deepspeed_stage_1_offload",

tests/tests_fabric/test_cli.py

@@ -71,8 +71,9 @@ def test_run_env_vars_strategy(_, strategy, monkeypatch, fake_script):
 def test_run_get_supported_strategies():
     """Test to ensure that when new strategies get added, we must consider updating the list of supported ones in the
     CLI."""
-    assert len(_get_supported_strategies()) == 7
+    assert len(_get_supported_strategies()) == 8
     assert "fsdp" in _get_supported_strategies()
+    assert "ddp_find_unused_parameters_true" in _get_supported_strategies()
 
 
 @pytest.mark.parametrize("strategy", ["ddp_spawn", "ddp_fork", "ddp_notebook", "deepspeed_stage_3_offload"])