name xpress variables and constraints

Summary:
We are adding explicit variable and constraint names to the xpress model formulation. This helps us achieve two objectives:

1. Make the problem formulation human readable (at least somewhat!) when we export it
2. Problem formulation becomes deterministic which enables us to hash it which can be used to cache the results

Reviewed By: micrain

Differential Revision: D72754854

fbshipit-source-id: 4bd085239f11e80f1cf477cb7b873ce4958f55c1

Author: mserturk

torchrec/distributed/planner/types.py

@@ -362,6 +362,18 @@ class ShardInfo:
     _index_in_shards: int
     _index_in_search_space: int
 
+    def __hash__(self) -> int:
+        return hash(
+            (
+                self.key,
+                self.sharding_type,
+                self.compute_kernel,
+                self.table_name,
+                self.shard,
+                self.module_fqn,
+            )
+        )
+
 
 class ShardingOption:
     """