[fix][minor] Change empty shard handling for OSS, do not rely on asserts (#460)

* change empty shard handling for OSS, do not rely on asserts
* code review

fairscale/optim/oss.py

@@ -140,13 +140,6 @@ class OSS(Optimizer):
                     param_group_rank["params"] = params
                     self._partition_parameters[rank].append(param_group_rank)
 
-            assert min(sum(len(pg["params"]) for pg in partition) for partition in self._partition_parameters) > 0, (
-                "One or more empty shards detected, the world size is too big or the model too small.\n"
-                + "Please reduce your world size if this is the model you would like to train\n"
-                + f"Current world size: {self.world_size}\n"
-                + "Current number of parameters: {}".format(sum(len(pg["params"]) for pg in self.param_groups))
-            )
-
         return self._partition_parameters
 
     @property
@@ -552,8 +545,11 @@ class OSS(Optimizer):
 
         for device in self.buckets.keys():
             for src_rank, bucket in enumerate(self.buckets[device]):
-                global_src_rank = self.get_global_rank(self.group, src_rank)
-                last_work_handle = dist.broadcast(tensor=bucket, src=global_src_rank, group=self.group, async_op=True)
+                if bucket.numel() > 0:
+                    global_src_rank = self.get_global_rank(self.group, src_rank)
+                    last_work_handle = dist.broadcast(
+                        tensor=bucket, src=global_src_rank, group=self.group, async_op=True
+                    )
 
         # Only check on the last handle, they're all inlined on the same CUDA stream
         if last_work_handle:
@@ -597,4 +593,5 @@ class OSS(Optimizer):
                     else:
                         self.buckets[device][dst_rank] = bucket
                 else:
-                    self.buckets[device].append(torch.zeros(1, device=device))
+                    # This rank has an empty shard, that's fine
+                    self.buckets[device].append(torch.zeros(0, device=device))

tests/optim/test_oss.py

@@ -262,19 +262,31 @@ def test_zero_grad():
     mp.spawn(run_test_zero_grad, args=(world_size, temp_file_name), nprocs=world_size, join=True)
 
 
-def run_test_catch_empty_shardd(rank, world_size, tempfile_name):
-    dist_init(rank, world_size, tempfile_name, backend="gloo")
+def run_test_empty_shard(rank, world_size, tempfile_name, backend):
+    dist_init(rank, world_size, tempfile_name, backend=backend)
     m = torch.nn.Linear(1, 1)
-    with pytest.raises(AssertionError):
-        _ = optim.OSS(m.parameters(), lr=0.1)
+    x = torch.rand(20, 1)
+
+    if torch.cuda.is_available():
+        m = m.to(rank)
+        x = x.to(rank)
+
+    o = optim.OSS(m.parameters(), lr=0.1)
+    y = m(x).sum()
+    y.backward()
+    o.step()
 
     dist.destroy_process_group()
 
 
-def test_empty_shard():
+@pytest.mark.parametrize("backend", ["gloo", "nccl"])
+def test_empty_shard(backend):
     world_size = 4
-
-    mp.spawn(run_test_catch_empty_shardd, args=(world_size, tempfile.mkstemp()[1]), nprocs=world_size, join=True)
+    if torch.cuda.is_available() and torch.cuda.device_count() < world_size:
+        world_size = min(world_size, torch.cuda.device_count())
+    if world_size == 1 or (backend == "nccl" and not torch.cuda.is_available()):
+        pytest.skip("Not enough GPUs to test with NCCL, or CUDA not present")
+    mp.spawn(run_test_empty_shard, args=(world_size, tempfile.mkstemp()[1], backend), nprocs=world_size, join=True)
 
 
 def run_test_step(rank, world_size, tempfile_name):