Support ExecutorchModule as a callable in pybindings (#1312)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1312

Users would be able to like to do the following, which is more consistent with how eager mode models and graph modules are callables.
```
model = _load_for_executorch(pte_file)
model(*inputs)
```
instead of
```
model = _load_for_executorch(pte_file)
model.forward(*inputs)
```

Reviewed By: chakriu, JacobSzwejbka

Differential Revision: D51673732

fbshipit-source-id: 2d754a0282929e28ff75cd34c95645087384f9f3