[Hot Fix] Give priority to plugins to set distributed mode, and then accelerator

CHANGELOG.md

@@ -24,6 +24,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed incorrect yield logic for the amp autocast context manager ([#6080](https://github.com/PyTorchLightning/pytorch-lightning/pull/6080))
 
 
+- Fixed priority of plugin/accelerator when setting distributed mode ([#6089](https://github.com/PyTorchLightning/pytorch-lightning/pull/6089))
+
+
 ## [1.2.0] - 2021-02-18
 
 ### Added

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -163,6 +163,9 @@ def handle_given_plugins(
 
         for plug in plugins:
             if isinstance(plug, str):
+                # Reset the distributed type as the user has overridden training type
+                # via the plugins argument
+                self._distrib_type = None
                 self.set_distributed_mode(plug)
 
             elif isinstance(plug, TrainingTypePlugin):
@@ -196,7 +199,6 @@ def handle_given_plugins(
                 )
 
         self._training_type_plugin = training_type
-        self._training_type_plugin = self.training_type_plugin
         self._precision_plugin = precision
         self._cluster_environment = cluster_environment or self.select_cluster_environment()
 

tests/accelerators/test_accelerator_connector.py

@@ -23,7 +23,14 @@
 from pytorch_lightning.accelerators.cpu import CPUAccelerator
 from pytorch_lightning.accelerators.gpu import GPUAccelerator
 from pytorch_lightning.callbacks import Callback
-from pytorch_lightning.plugins import DDP2Plugin, DDPPlugin, DDPSpawnPlugin, PrecisionPlugin, SingleDevicePlugin
+from pytorch_lightning.plugins import (
+    DDP2Plugin,
+    DDPPlugin,
+    DDPShardedPlugin,
+    DDPSpawnPlugin,
+    PrecisionPlugin,
+    SingleDevicePlugin,
+)
 from pytorch_lightning.plugins.environments import ClusterEnvironment, SLURMEnvironment, TorchElasticEnvironment
 from tests.helpers.boring_model import BoringModel
 
@@ -378,3 +385,18 @@ def on_fit_start(self, trainer, pl_module):
 
     with pytest.raises(SystemExit):
         trainer.fit(model)
+
+
+@pytest.mark.parametrize(
+    ["accelerator", "plugin"],
+    [('ddp_spawn', 'ddp_sharded'), (None, 'ddp_sharded')],
+)
+def test_plugin_accelerator_choice(accelerator, plugin):
+    """
+    Ensure that when a plugin and accelerator is passed in, that the plugin takes precedent.
+    """
+    trainer = Trainer(accelerator=accelerator, plugins=plugin, num_processes=2)
+    assert isinstance(trainer.accelerator.training_type_plugin, DDPShardedPlugin)
+
+    trainer = Trainer(plugins=plugin, num_processes=2)
+    assert isinstance(trainer.accelerator.training_type_plugin, DDPShardedPlugin)