Fix BoundaryPooling tracing (#713)

Summary:
Pull Request resolved: #713

Array slicing using the results of a .size() will store the slice index as constant in the trace. In this case we don't even need to tracing to work nicely because we can just record a constant -1 for the slicing.

Reviewed By: liaimi

Differential Revision: D15932989

fbshipit-source-id: 2fc05cf953b091f15ba0bcd97c90ffcc01740aec

pytext/models/representations/pooling.py

@@ -91,18 +91,17 @@ def __init__(self, config: Config, n_input: int) -> None:
     def forward(
         self, inputs: torch.Tensor, seq_lengths: torch.Tensor = None
     ) -> torch.Tensor:
-        max_len = inputs.size()[1]
         if self.boundary_type == "first":
             return inputs[:, 0, :]
         elif self.boundary_type == "last":
             # could only have the bos values if add_bos or add_eos as False
             # should not reach here if the eos is not added.
-            assert max_len > 1
-            return inputs[:, max_len - 1, :]
+            assert inputs.size()[1] > 1
+            return inputs[:, -1, :]
         elif self.boundary_type == "firstlast":
-            assert max_len > 1
+            assert inputs.size()[1] > 1
             # merge from embed_dim into 2*emded_dim
-            return torch.cat((inputs[:, 0, :], inputs[:, max_len - 1, :]), dim=1)
+            return torch.cat((inputs[:, 0, :], inputs[:, -1, :]), dim=1)
         else:
             raise Exception("Unknown configuration type {}".format(self.boundary_type))