ZeRO-2: Handle gradients of empty partitions (#275)

* Load non-DeepSpeed checkpoints into ZeRO optimizer

* Handle parameters smaller than DP

* Formatting fixes

* Handle empty partitions

* Fix perf bug
Co-authored-by: Jeff Rasley <jerasley@microsoft.com>

deepspeed/pt/deepspeed_zero_optimizer.py

@@ -403,6 +403,7 @@ class FP16_DeepSpeedZeroOptimizer(object):
                 self.is_grad_computed[i][partition_id] = {}
                 self.grad_partition_insertion_offset[i][partition_id] = {}
                 self.grad_start_offset[i][partition_id] = {}
+                self.total_grads_in_partition[i][partition_id] = 0
                 self.initialize_gradient_partition(i, param_group, partition_id)
                 self.is_partition_reduced[i][partition_id] = False
                 self.first_param_index_in_partition[i][
@@ -429,6 +430,8 @@ class FP16_DeepSpeedZeroOptimizer(object):
                 self.params_in_partition[i],
                 self.first_offset[i],
                 self.partition_size[i],
+                dtype=torch.half,
+                device=torch.cuda.current_device(),
                 return_tensor_list=True)
 
         self._release_ipg_buffers()
@@ -1014,6 +1017,8 @@ class FP16_DeepSpeedZeroOptimizer(object):
                            tensor_list,
                            first_offset,
                            partition_size,
+                           dtype,
+                           device,
                            return_tensor_list=False):
         flat_tensor_list = []
         current_size = 0
@@ -1050,8 +1055,8 @@ class FP16_DeepSpeedZeroOptimizer(object):
         if current_size < partition_size:
             flat_tensor_list.append(
                 torch.zeros(int(partition_size - current_size),
-                            dtype=tensor_list[0].dtype,
-                            device=tensor_list[0].device))
+                            dtype=dtype,
+                            device=device))
 
         if return_tensor_list:
             return flat_tensor_list
@@ -1135,7 +1140,6 @@ class FP16_DeepSpeedZeroOptimizer(object):
         for group in self.single_partition_of_fp32_groups:
             group.grad = None
 
-        for i in range(len(norm_groups)):
         for fp16_partitions, fp32_partition in zip(self.parallel_partitioned_fp16_groups, self.single_partition_of_fp32_groups):
             fp16_partitions[partition_id].data.copy_(fp32_partition.data)
         timers('optimizer_step').stop()

tests/unit/test_fp16.py

@@ -353,7 +353,7 @@ def test_zero_allow_untested_optimizer(tmpdir, zero_stage):
     _test_zero_allow_untested_optimizer(args)
 
 
-@pytest.mark.parametrize("zero_stage", [1])
+@pytest.mark.parametrize("zero_stage", [1, 2])
 def test_zero_empty_partition(tmpdir, zero_stage):
     config_dict = {
         "train_micro_batch_size_per_gpu": 1,