[autoparallel] added compute resharding costs for node handler (#1662)

colossalai/auto_parallel/solver/op_handler/node_handler.py

 from abc import ABC, abstractmethod
 from torch.fx.node import Node
 from colossalai.device.device_mesh import DeviceMesh
+from colossalai.tensor.shape_consistency import ShapeConsistencyManager
 from typing import Dict, List
-from ..sharding_strategy import ShardingStrategy_V2, StrategiesVector, OperationData
+from ..sharding_strategy import ShardingStrategy_V2, StrategiesVector, OperationData, TrainCycleItem
 from ..strategy import StrategyGenerator_V2
 
 
@@ -28,13 +29,53 @@ class NodeHandler(ABC):
         self.device_mesh = device_mesh
         self.strategies_vector = strategies_vector
 
-    def register_strategy(self) -> StrategiesVector:
+    def update_resharding_cost(self, strategy: ShardingStrategy_V2) -> None:
+        """
+        Compute the resharding costs and save the costs in the ShardingStrategy object.
+        """
+        # TODO: test this function when other handlers are ready
+        resharding_costs = {}
+        shape_consistency_manager = ShapeConsistencyManager()
+        for node in self.predecessor_node:
+            node_name = str(node)
+
+            # get the sharding specs for this node generated
+            # in its own node handler
+            assert hasattr(node, 'strategies_vector'), \
+                f'The predecessor node {node_name} has no strategy vector to compute the resharding cost.'
+            prev_strategy_vector = node.strategies_vector
+            prev_sharding_specs = [strategy.get_sharding_spec_by_name(node_name) for strategy in prev_strategy_vector]
+
+            # get the current sharding spec generated by this node handler
+            op_data = strategy.get_op_data_by_name(node_name)
+            current_sharding_spec = strategy.sharding_specs[op_data]
+
+            # create data structrure to store costs
+            if op_data not in resharding_costs:
+                resharding_costs[op_data] = {}
+
+            # for each sharding spec generated by the predecessor's node handler
+            # compute the resharding cost to switch to the sharding spec generated
+            # by the current node handler
+            for prev_sharding_spec in prev_sharding_specs:
+                fwd_cost = shape_consistency_manager.shape_consistency(prev_sharding_spec, current_sharding_spec)
+                bwd_cost = shape_consistency_manager.shape_consistency(current_sharding_spec, prev_sharding_spec)
+                resharding_cost = TrainCycleItem(fwd=fwd_cost, bwd=bwd_cost, total=fwd_cost + bwd_cost)
+                resharding_costs[op_data][prev_sharding_spec] = resharding_cost
+        strategy.resharding_costs = resharding_costs
+
+    def register_strategy(self, compute_resharding_cost: bool = False) -> StrategiesVector:
         """
         Register different sharding strategies for the current node.
         """
         strategy_generators = self.get_strategy_generator()
         for generator in strategy_generators:
             strategies = generator.generate()
+
+            # compute the resharding costs based on the previous node
+            # strategies if specified
+            if compute_resharding_cost:
+                strategies = list(map(self.update_resharding_cost, strategies))
             self.strategies_vector.extend(strategies)
 
         strategies_vector = map(self.post_process, self.strategies_vector)

colossalai/auto_parallel/solver/sharding_strategy.py

@@ -129,6 +129,7 @@ class ShardingStrategy_V2:
     memory_cost: TrainCycleItem = None
     input_resharding_costs: Dict[OperationData, List[float]] = None
     communication_actions: Dict[OperationData, CommSpec] = None
+    resharding_costs: Dict[OperationData, Dict[ShardingSpec, TrainCycleItem]] = None
 
     @property
     def input_sharding_specs(self) -> Dict[OperationData, ShardingSpec]:
@@ -153,6 +154,18 @@ class ShardingStrategy_V2:
         specs = {k: v for k, v in self.sharding_specs.items() if k.type == operation_data_type}
         return specs
 
+    def get_op_data_by_name(self, name: str):
+        for op_data in self.sharding_specs.keys():
+            if op_data.name == name:
+                return op_data
+        raise KeyError(f"Could not find the OperationData with name {name}")
+
+    def get_sharding_spec_by_name(self, name: str):
+        for op_data, sharding_spec in self.sharding_specs.items():
+            if op_data.name == name:
+                return sharding_spec
+        raise KeyError(f"Could not find the ShardingSpec for OperationData with name {name}")
+
 
 class StrategiesVector(list):
     '''