[fx][split] Copy node metadata for placeholders (#107981)

- Follow-up to #107248 which copies metadata for placeholder nodes in the top-level FX graph
- Currently, top-level placeholders do not have their metadata copied over, causing loss of `TensorMetadata` in some `torch.compile` backends

Fixes pytorch/TensorRT#2258
Pull Request resolved: #107981
Approved by: https://github.com/angelayi

torch/fx/passes/split_utils.py

@@ -160,6 +160,7 @@ def flatten(x: torch.fx.node.Argument) -> NodeList:
         # Placeholders in the original graph get copied to main graph.
         if node.op == "placeholder":
             main_remapping[node] = main_g.placeholder(node.name, type_expr=node.type)
+            main_remapping[node].meta = copy.copy(node.meta)
             continue
 
         # Get_attr nodes are ignored because we are not tagging them.