Make grad accum steps mutable on the Accelerator object (#1233)

* Make grad accum steps mutable

* Reset state

src/accelerate/accelerator.py

@@ -819,10 +819,18 @@ class Accelerator:
     def sync_gradients(self):
         return self.gradient_state.sync_gradients
 
+    @sync_gradients.setter
+    def sync_gradients(self, sync_gradients):
+        self.gradient_state.sync_gradients = sync_gradients
+
     @property
     def gradient_accumulation_steps(self):
         return self.gradient_state.num_steps
 
+    @gradient_accumulation_steps.setter
+    def gradient_accumulation_steps(self, gradient_accumulation_steps):
+        self.gradient_state.plugin_kwargs.update({"num_steps": gradient_accumulation_steps})
+
     @contextmanager
     def accumulate(self, model):
         """

tests/test_accelerator.py

@@ -9,7 +9,7 @@ from torch.utils.data import DataLoader, TensorDataset
 
 from accelerate import infer_auto_device_map, init_empty_weights
 from accelerate.accelerator import Accelerator
-from accelerate.state import PartialState
+from accelerate.state import GradientState, PartialState
 from accelerate.test_utils import require_multi_gpu, slow
 from accelerate.test_utils.testing import AccelerateTestCase, require_cuda
 from accelerate.utils import patch_environment
@@ -43,6 +43,18 @@ class AcceleratorTester(AccelerateTestCase):
         with self.assertRaises(ValueError):
             _ = Accelerator(cpu=True)
 
+    def test_mutable_states(self):
+        accelerator = Accelerator()
+        state = GradientState()
+        assert state.num_steps == 1
+        accelerator.gradient_accumulation_steps = 4
+        assert state.num_steps == 4
+
+        assert state.sync_gradients is True
+        accelerator.sync_gradients = False
+        assert state.sync_gradients is False
+        GradientState._reset_state()
+
     def test_prepared_objects_are_referenced(self):
         accelerator = Accelerator()
         model, optimizer, scheduler, train_dl, valid_dl = create_components()