Handle None input tensors in lowering if they are eventually not used (#85)

Summary:
Pull Request resolved: https://github.com/pytorch/fx2trt/pull/85

Corner case.

Reviewed By: khabinov

Differential Revision: D36727701

fbshipit-source-id: f2a105d70a1f1a04395ae08e73ae614a080947a2

Author: Yinghai Lu

fx/lower.py

@@ -230,7 +230,10 @@ def __call__(
             == LowerPrecision.FP16
         ):
             module.half()
-            inputs = tuple(x.half() if x.dtype == torch.float32 else x for x in inputs)
+            inputs = tuple(
+                x.half() if x is not None and x.dtype == torch.float32 else x
+                for x in inputs
+            )
         pm = self.lower_pass_manager_builder.build_lower_pipeline(inputs)
 
         lower_result = pm(module)