Add ddp_find_unused_parameters_true alias in Fabric's DDPStrategy (#…

…20125)

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"])