register_buffer for DLRM.triu_indices to make it exportable (#1670)

Summary:
Pull Request resolved: #1670

we need to register buffer for `triu_indices` to make DLRM "exportable" -- thus to generate PT2 IR.

Reviewed By: joshuadeng

Differential Revision: D53233580

fbshipit-source-id: 451c70ea5e3433756b3a351f74e9eedc99cfca0b

Author: wilson100hong

torchrec/models/dlrm.py

@@ -183,8 +183,10 @@ class InteractionArch(nn.Module):
     def __init__(self, num_sparse_features: int) -> None:
         super().__init__()
         self.F: int = num_sparse_features
-        self.triu_indices: torch.Tensor = torch.triu_indices(
-            self.F + 1, self.F + 1, offset=1
+        self.register_buffer(
+            "triu_indices",
+            torch.triu_indices(self.F + 1, self.F + 1, offset=1),
+            persistent=False,
         )
 
     def forward(