[examples] make run scripts executable
For consistently and convenience of not needing to type `python` to run a script this PR adds to all `examples/*/run_*py` scripts a python shebang and make the scripts executable.

Was done with:
```
perl -0777 -pi -e 's|^|#!/usr/bin/env python\n|' */run_*.py
chmod a+x */run_*.py
```

@patil-suraj , @sgugger 
Totally!

Just this one:
```
/templates/adding_a_new_example_script/{{cookiecutter.directory_name}}/run_{{cookiecutter.example_shortcut}}.py
```
correct?

Any other scripts that I didn't catch with `examples/*/run_*.py`?
<|||||>I don't think so, we should be good.