[pytorch][te] Don't start TE fusion groups with an unknown-typed result (#47884)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47884
We need to know output types of everything in a fusion group to ensure
that we generate correctly-typed tensors. We were incorrectly starting a
fusion group with an unknown-typed output.
Test Plan:
New unit tests:
```
buck test //caffe2/test:jit //caffe2/test/cpp/tensorexpr:tensorexpr
```
Reviewed By: eellison
Differential Revision: D24932786
fbshipit-source-id: 83978a951f32c1207bbc3555a7d3bd94fe4e70fb
