[Distributed] create model on meta device (#1227)

Author: kwen2501

dist_run.py

@@ -328,15 +328,26 @@ def main(args):
     config.stage_idx = pp_rank
     config.n_stages = pp_degree
 
-    with device:
+    with torch.device("meta"):
         # TODO: we should create model instead of Transformer
         model = Transformer(config)
 
     # Distribute model on TP mesh
+    # (Surprisingly, this works even though model is on meta device and mesh is of
+    # cuda devices)
     model.distribute(tp_mesh)
     if rank == 0:
         logger.info(f"Model: {model}")
 
+    # Load weights
+    logger.info(f"Loading weights for {pp_rank=} on {device=}")
+    with CUDATrackTime() as timer:
+        _load_model_weights(model, distribution, device=device, model_config=config)
+
+    logger.info(
+        f"{color.green}Total weight loading time: {timer.get_time()} {timer.unit} for rank {rank}{color.reset}"
+    )
+
     # Batch size. Since we push batches dynamically through the pipeline rather
     # than chunking them, this is effectively micro-batch size in pipeline
     # sense. Thus it is interchangeable with micro-batch size below.
@@ -352,17 +363,8 @@ def main(args):
     # lanes.
     # TODO: bump up the lane count
     pipeline_lanes = 1
-    model.setup_caches(batch_size, seqlen_prefill, cache_lanes=pipeline_lanes)
-
-    # Load weights
-    logger.info(f"Loading weights for {pp_rank=} on {device=}")
-    with CUDATrackTime() as timer:
-        _load_model_weights(model, distribution, device=device, model_config=config)
-        model.to(device)
-
-    logger.info(
-        f"{color.green}Total weight loading time: {timer.get_time()} {timer.unit} for rank {rank}{color.reset}"
-    )
+    with device:
+        model.setup_caches(batch_size, seqlen_prefill, cache_lanes=pipeline_lanes)
 
     # info on stage size and params
     stage_size = get_module_size(model)

torchchat/distributed/dtensor_utils.py

@@ -8,24 +8,17 @@
 logger = SingletonLogger.get_logger()
 
 
-
-def is_dtensor(tensor):
-    """Check if a tensor is a DTensor by class or has a placements attribute (not sure if we want to use attr check)"""
-    return isinstance(tensor, DTensor) or hasattr(tensor, "placements")
-
-
-def load_into_dtensor(weight_tensor, model_dtensor):
+def convert_to_dtensor(weight_tensor, dtensor_template):
     """Adjust a loaded tensor to match the shape/placement of the model DTensor and copy the data into it"""
-    weight_tensor = weight_tensor.to(model_dtensor.device)
 
-    if weight_tensor.shape != model_dtensor.shape:
+    if weight_tensor.shape != dtensor_template.shape:
         raise ValueError(
             f"Shape mismatch: weight tensor shape {weight_tensor.shape} "
-            f"doesn't match DTensor shape {model_dtensor.shape}"
+            f"doesn't match DTensor shape {dtensor_template.shape}"
         )
 
-    placements = model_dtensor.placements
-    mesh = model_dtensor.device_mesh
+    placements = dtensor_template.placements
+    mesh = dtensor_template.device_mesh
     mesh_dims = mesh.ndim
 
     for placement in placements:

torchchat/distributed/safetensor_utils.py

@@ -13,8 +13,8 @@
 from torch.nn import Module
 from typing import Dict, Tuple, Set, Optional
 
-
-from torchchat.distributed.dtensor_utils import is_dtensor, load_into_dtensor
+from torch.distributed._tensor import DTensor
+from torchchat.distributed.dtensor_utils import convert_to_dtensor
 
 
 _DEFAULT_SAFETENSOR_FILE_NAME = "model.safetensors.index.json"
@@ -284,9 +284,7 @@ def update_state_dict(
                 continue
 
             checkpoint_tensor = checkpoint[old_param]
-            stage_tensor = state_dict[param]
-
-            stage_is_dtensor = is_dtensor(stage_tensor)
+            model_tensor = state_dict[param]
 
             if "wq" in param:
                 checkpoint_tensor = permute_weight_to_attn_heads(
@@ -297,17 +295,16 @@ def update_state_dict(
                     checkpoint_tensor, num_local_heads, head_dim, dim
                 )
 
+            # Move checkpoint tensor to desired device
+            checkpoint_tensor = checkpoint_tensor.to(device)
+
             # here we need to check if the tensor is a DTensor and if so, adjust the
             # shape and placement to match the model DTensor.
-            if stage_is_dtensor:
-                model_tensor = load_into_dtensor(checkpoint_tensor, stage_tensor)
-                # logger.info(f"DTensor: Loaded {param} into {model_tensor=}")
-                state_dict[param] = model_tensor
+            if isinstance(model_tensor, DTensor):
+                state_dict[param] = convert_to_dtensor(checkpoint_tensor, model_tensor)
                 count_dtensors_loaded += 1
-
             else:
                 # regular tensor, just update directly
-                checkpoint_tensor = checkpoint_tensor.to(device)
                 state_dict[param] = checkpoint_tensor
 
             # ensure matching dtypes