Dynamically determine the version of the pip package. (#3475)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3475

Use the logic from
https://github.com/pytorch/torcharrow/blob/15a7f7124d4c73c8c541547aef072264baab63b7/setup.py#L21
to play nicely with the pytorch ecosystem CI build environment.

Test Plan:
```
$ ./install_requirements.sh
...
Successfully installed executorch-0.2.0a0+1ba292a

$ python
>>> from executorch import version
>>> version.__version__
'0.2.0a0+1ba292a'
>>> version.git_version
'1ba292ae4071c4eede8ea14e8f10ffd973a085b4'
>>> ^D

$ grep Version /home/dbort/.conda/envs/executorch-tmp/lib/python3.10/site-packages/executorch-0.2.0a0+1ba292a.dist-info/METADATA
Metadata-Version: 2.1
Version: 0.2.0a0+1ba292a
```

Temporarily commented out the call to `setup()` in `setup.py` then
imported it.

```
$ python
>>> from setup import Version
>>> Version.string
'0.2.0a0+1ba292a'
>>> Version.git_hash
'1ba292ae4071c4eede8ea14e8f10ffd973a085b4'
>>> Version.write_to_python_file("/tmp/version.py")
>>> ^D
$ cat /tmp/version.py
from typing import Optional
__all__ = ["__version__", "git_version"]
__version__ = "0.2.0a0+1ba292a"
git_version: Optional[str] = '1ba292ae4071c4eede8ea14e8f10ffd973a085b4'
```

```
$ BUILD_VERSION="5.5.5" python
>>> from setup import Version
>>> Version.string
'5.5.5'
```

imported-using-ghimport

Reviewed By: mikekgfb

Differential Revision: D56857484

Pulled By: dbort

fbshipit-source-id: c9af223c591fed6fb5307fa5ea8c2371dd38f4d8