fix bug in deep fp8 zero (#141)

**Description**
Fix 2 bugs in fp8 zero optimizer.
MS-AMP will crash when:
- there are param groups which don't have high precision parameter
-  partition 0 has no parameters.

Author: tocean

msamp/deepspeed/runtime/zero/fp8_stage_1_and_2.py

@@ -34,6 +34,15 @@ def __init__(self, init_optimizer, *args, **kwargs):    # noqa: C901
             **kwargs: Arbitrary keyword arguments.
         """
         self.fp8_param_groups = []
+        dtype = torch.float16
+        for pg in init_optimizer.param_groups:
+            for p in pg['params']:
+                if p.requires_grad and not isinstance(p, ScalingTensor):
+                    dtype = p.dtype
+                    break
+
+        fake_param = torch.nn.parameter.Parameter(torch.zeros((), dtype=dtype))
+        fake_index = 0
         for pg in init_optimizer.param_groups:
             fp8_params = []
             hp_params = []
@@ -45,6 +54,13 @@ def __init__(self, init_optimizer, *args, **kwargs):    # noqa: C901
                 else:
                     hp_params.append(p)
             self.fp8_param_groups.append(fp8_params)
+            # DeepSpeedZeroOptimizer will crash if there is no parameters in any parameter group,
+            # so add a fake parameter.
+            if len(hp_params) == 0:
+                param_names = args[0]
+                param_names[fake_param] = 'fake_' + str(fake_index)
+                fake_index += 1
+                hp_params.append(fake_param)
             pg['params'] = hp_params
 
         assert len(self.fp8_param_groups) == len(init_optimizer.param_groups)
@@ -139,9 +155,14 @@ def _pad_and_flat(self, values_partitions, group_fp8_mems, group_id):
             torch.Tensor: flat fp8 groups.
         """
         partition_size = dist.get_world_size(group=self.dp_process_group)
-        ref_value = values_partitions[0][0]
-        dtype = ref_value.dtype
-        assert all(v.dtype == dtype for v in chain(*values_partitions))
+        ref_value = None
+        for partition in values_partitions:
+            if len(partition) > 0:
+                ref_value = partition[0]
+                break
+        if ref_value is not None:
+            dtype = ref_value.dtype
+            assert all(v.dtype == dtype for v in chain(*values_partitions))
 
         align = self.fp8_nccl_start_alignment_factor
         max_flat_numels = max(group_fp8_mems)
@@ -777,12 +798,12 @@ def all_gather_fp8_metas(self):
                 continue
             partition_size = len(params_partitions)
             scale_invs_partitions = [[p.meta.scale_inv for p in ps] for ps in params_partitions]
-            ref_scale = scale_invs_partitions[0][0]
             align = self.fp8_nccl_start_alignment_factor
             max_flat_numels = (max_flat_numels + align - 1) // align * align
             for pi in range(partition_size):
                 pad = max_flat_numels - numels[pi]
-                scale_invs_partitions[pi].append(ref_scale.new_empty((pad, )))
+                scale_invs_partitions[pi].append(torch.empty((pad, ), dtype=torch.float32, device='cuda'))
+
             scales = list(chain(*scale_invs_partitions))
             scale_invs_groups.append(scales)
             flat = _flatten_dense_tensors(scales)