Replace deprecated DDP accelerator with ddp_find_unused_parameters_false

Summary: We proactively remove references to the deprecated DDP accelerator to prepare for the breaking changes following the release of PyTorch Lighting 1.6 (see T112240890).

Differential Revision: D34295318

fbshipit-source-id: 7b2245ca9c7c2900f510722b33af8d8eeda49919

tests/runner/test_runner_lightning_quantization.py

@@ -189,7 +189,7 @@ class TestQuantizationAwareTraining(unittest.TestCase):
         num_epochs = 2
         qat = QuantizationAwareTraining()
         trainer = Trainer(
-            accelerator="ddp_cpu",
+            accelerator="cpu",
             num_processes=1,
             default_root_dir=os.path.join(root_dir, "quantized"),
             checkpoint_callback=False,