Publicly link libtorch.so into pybind portable_lib

Summary:
Building from source on Google Colab reveals this issue that:

```
from executorch.extension.pybindings import portable_lib
```

Result in undefined symbols in libtorch.so. This doesn't repro in GH CI jobs probably because of cmake/compiler version difference.

Linking `libtorch.so` directly into the pybind library `portable_lib` fixes this issue.

Reviewed By: dbort

Differential Revision: D54128075

fbshipit-source-id: 18b01a0af925c00fc690c9fbcdc4ffcd13adc5d1