Add Support for FakeProcessGroup (pytorch#1877)

Summary:
X-link: pytorch/pytorch#133039

Pull Request resolved: pytorch#1877

# context
* use FakeProcessGroup to mimic the multi-process tests
* can use `_test_compile_fake_pg_fn` as the single-process VB compile test
```
from torchrec.distributed.tests.test_pt2_multiprocess import _test_compile_fake_pg_fn
_test_compile_fake_pg_fn(
    rank=0,
    world_size=2,
)
```

reference: D59637444

Differential Revision: D56124045

torchrec/distributed/batched_embedding_kernel.py

@@ -736,13 +736,17 @@ def __init__(
             self._emb_module,
             pg,
         )
-        self._param_per_table: Dict[str, nn.Parameter] = dict(
-            _gen_named_parameters_by_table_ssd(
-                emb_module=self._emb_module,
-                table_name_to_count=self.table_name_to_count.copy(),
-                config=self._config,
-                pg=pg,
+        self._param_per_table: Dict[str, nn.Parameter] = (
+            dict(
+                _gen_named_parameters_by_table_ssd(
+                    emb_module=self._emb_module,
+                    table_name_to_count=self.table_name_to_count.copy(),
+                    config=self._config,
+                    pg=pg,
+                )
             )
+            if pg._get_backend_name() != "fake"
+            else {}
         )
         self.init_parameters()
 
@@ -1308,13 +1312,17 @@ def __init__(
             self._emb_module,
             pg,
         )
-        self._param_per_table: Dict[str, TableBatchedEmbeddingSlice] = dict(
-            _gen_named_parameters_by_table_fused(
-                emb_module=self._emb_module,
-                table_name_to_count=self.table_name_to_count.copy(),
-                config=self._config,
-                pg=pg,
+        self._param_per_table: Dict[str, TableBatchedEmbeddingSlice] = (
+            dict(
+                _gen_named_parameters_by_table_fused(
+                    emb_module=self._emb_module,
+                    table_name_to_count=self.table_name_to_count.copy(),
+                    config=self._config,
+                    pg=pg,
+                )
             )
+            if pg._get_backend_name() != "fake"
+            else {}
         )
         self.init_parameters()
 

torchrec/distributed/comm_ops.py

@@ -481,7 +481,7 @@ def variable_batch_alltoall_pooled(
     if group is None:
         group = dist.distributed_c10d._get_default_group()
 
-    if dist.get_world_size(group) <= 1:
+    if group.size() <= 1:
         return NoWait(a2a_pooled_embs_tensor)
 
     a2ai = VariableBatchAll2AllPooledInfo(
@@ -509,7 +509,7 @@ def variable_batch_all2all_pooled_sync(
     my_rank = pg.rank()
 
     # get input splits
-    world_size = dist.get_world_size(pg)
+    world_size = pg.size()
     input_split_sizes = [0 for _ in range(world_size)]
     if a2ai.batch_size_per_rank_per_feature:
         for i in range(world_size):
@@ -559,14 +559,25 @@ def variable_batch_all2all_pooled_sync(
         ]
 
     with record_function("## alltoall_fwd_single ##"):
-        sharded_output_embeddings = torch.ops.torchrec.all_to_all_single(
-            sharded_input_embeddings,
-            output_split_sizes,
-            input_split_sizes,
-            pg_name(pg),
-            pg.size(),
-            get_gradient_division(),
-        )
+        if pg._get_backend_name() == "fake":
+            sharded_output_embeddings = torch.empty(
+                sum(output_split_sizes),
+                device=sharded_input_embeddings.device,
+                dtype=sharded_input_embeddings.dtype,
+            )
+            s0 = sharded_output_embeddings.size(0)
+            # Bad assumption that our rank GE than other
+            torch._check(s0 <= sharded_input_embeddings.size(0))
+            sharded_output_embeddings.copy_(sharded_input_embeddings[:s0])
+        else:
+            sharded_output_embeddings = torch.ops.torchrec.all_to_all_single(
+                sharded_input_embeddings,
+                output_split_sizes,
+                input_split_sizes,
+                pg_name(pg),
+                pg.size(),
+                get_gradient_division(),
+            )
 
     if a2ai.codecs is not None:
         codecs = none_throws(a2ai.codecs)

torchrec/distributed/dist_data.py

@@ -239,12 +239,25 @@ def __init__(
             # https://github.com/pytorch/pytorch/issues/122788
             with record_function("## all2all_data:kjt splits ##"):
                 input_tensor = torch.stack(input_tensors, dim=1).flatten()
-                self._output_tensor = dist._functional_collectives.all_to_all_single(
-                    input_tensor,
-                    output_split_sizes=None,
-                    input_split_sizes=None,
-                    group=pg,
-                )
+                if pg._get_backend_name() == "fake":
+                    self._output_tensor = torch.empty(
+                        [self.num_workers * len(input_tensors)],
+                        device=input_tensors[0].device,
+                        dtype=input_tensors[0].dtype,
+                    )
+
+                    self._output_tensor = input_tensor[
+                        : input_tensor.size(0) // 2
+                    ].repeat(2)
+                else:
+                    self._output_tensor = (
+                        dist._functional_collectives.all_to_all_single(
+                            input_tensor,
+                            output_split_sizes=None,
+                            input_split_sizes=None,
+                            group=pg,
+                        )
+                    )
             # To avoid hasattr in _wait_impl to check self._splits_awaitable
             # pyre-ignore
             self._splits_awaitable = None
@@ -342,6 +355,7 @@ def __init__(
         self._output_tensors: List[torch.Tensor] = []
         self._awaitables: List[dist.Work] = []
         self._world_size: int = self._pg.size()
+        rank = self._pg.rank()
 
         for input_split, output_split, input_tensor, label in zip(
             input_splits,
@@ -353,12 +367,28 @@ def __init__(
                 # TODO(ivankobzarev) Remove this dynamo condition once dynamo functional collectives remapping does not emit copy_
                 # https://github.com/pytorch/pytorch/issues/122788
                 with record_function(f"## all2all_data:kjt {label} ##"):
-                    output_tensor = dist._functional_collectives.all_to_all_single(
-                        input_tensor,
-                        output_split,
-                        input_split,
-                        pg,
-                    )
+                    if self._pg._get_backend_name() == "fake":
+                        output_tensor = torch.empty(
+                            sum(output_split),
+                            device=self._device,
+                            dtype=input_tensor.dtype,
+                        )
+                        _l = sum(output_split[:rank])
+                        _r = _l + output_split[rank]
+                        torch._check(_r < input_tensor.size(0))
+                        torch._check(_l < input_tensor.size(0))
+                        torch._check(_l <= _r)
+                        torch._check(2 * (_r - _l) == output_tensor.size(0))
+                        output_tensor.copy_(
+                            input_tensor[_l:_r].repeat(self._world_size)
+                        )
+                    else:
+                        output_tensor = dist._functional_collectives.all_to_all_single(
+                            input_tensor,
+                            output_split,
+                            input_split,
+                            pg,
+                        )
                     self._output_tensors.append(output_tensor)
             else:
                 output_tensor = torch.empty(
@@ -599,7 +629,7 @@ def forward(
 
         with torch.no_grad():
             assert len(input.keys()) == sum(self._splits)
-            rank = dist.get_rank(self._pg)
+            rank = self._pg.rank()
             local_keys = input.keys()[
                 self._splits_cumsum[rank] : self._splits_cumsum[rank + 1]
             ]

torchrec/distributed/embeddingbag.py

@@ -657,7 +657,9 @@ def __init__(
                     broadcast_buffers=True,
                     static_graph=True,
                 )
-        self._initialize_torch_state()
+
+        if env.process_group._get_backend_name() != "fake":
+            self._initialize_torch_state()
 
         # TODO[zainhuda]: support module device coming from CPU
         if module.device not in ["meta", "cpu"] and module.device.type not in [