Add deprecation warning & test for distributed_backend flag (#8575)

Co-authored-by: Adrian Wälchli <[email protected]>

Author: kaushikb11

CHANGELOG.md

@@ -173,6 +173,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Deprecated the `Trainer.disable_validation` property in favor of `not Trainer.enable_validation` ([#8291](https://github.com/PyTorchLightning/pytorch-lightning/pull/8291))
 - Deprecated `mode` parameter in `ModelSummary` in favor of `max_depth` ([#8062](https://github.com/PyTorchLightning/pytorch-lightning/pull/8062))
 - Deprecated `reload_dataloaders_every_epoch` argument of `Trainer` in favor of `reload_dataloaders_every_n_epochs` ([#5043](https://github.com/PyTorchLightning/pytorch-lightning/pull/5043))
+- Deprecated `distributed_backend` argument for `Trainer` ([#8575](https://github.com/PyTorchLightning/pytorch-lightning/pull/8575))
 
 
 ### Removed

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -88,6 +88,7 @@ def __init__(
         tpu_cores,
         ipus,
         distributed_backend,
+        accelerator,
         gpus,
         gpu_ids,
         num_nodes,
@@ -105,6 +106,13 @@ def __init__(
         self._distrib_type = None
         self._accelerator_type = None
 
+        if distributed_backend is not None:
+            rank_zero_deprecation(
+                f"`Trainer(distributed_backend={distributed_backend})` has been deprecated and will be removed in v1.5."
+                f" Use `Trainer(accelerator={distributed_backend})` instead."
+            )
+        distributed_backend = distributed_backend or accelerator
+
         self.num_processes = num_processes
         self.devices = devices
         # `gpus` is the input passed to the Trainer, whereas `gpu_ids` is a list of parsed gpu ids.

pytorch_lightning/trainer/trainer.py

@@ -342,7 +342,7 @@ def __init__(
         super().__init__()
         Trainer._log_api_event("init")
         self.state = TrainerState()
-        distributed_backend = distributed_backend or accelerator
+
         gpu_ids, tpu_cores = self._parse_devices(gpus, auto_select_gpus, tpu_cores)
 
         # init connectors
@@ -357,6 +357,7 @@ def __init__(
             tpu_cores,
             ipus,
             distributed_backend,
+            accelerator,
             gpus,
             gpu_ids,
             num_nodes,

tests/deprecated_api/test_remove_1-5.py

@@ -347,3 +347,8 @@ def test_v1_5_0_deepspeed_cpu_offload(tmpdir, params):
 
     with pytest.deprecated_call(match="is deprecated since v1.4 and will be removed in v1.5"):
         DeepSpeedPlugin(**params)
+
+
+def test_v1_5_0_distributed_backend_trainer_flag():
+    with pytest.deprecated_call(match="has been deprecated and will be removed in v1.5."):
+        Trainer(distributed_backend="ddp_cpu")