[examples] make run scripts executable (#10037)

* make executable

* make executable

* same for the template

* cleanup

examples/text-classification/run_tf_text_classification.py

+#!/usr/bin/env python
 # coding=utf-8
 # Copyright 2020 The HuggingFace Team. All rights reserved.
 #

examples/text-classification/run_xnli.py

+#!/usr/bin/env python
 # coding=utf-8
 # Copyright 2018 The Google AI Language Team Authors and The HuggingFace Inc. team.
 # Copyright (c) 2018, NVIDIA CORPORATION.  All rights reserved.

examples/text-generation/run_generation.py

-#!/usr/bin/env python3
+#!/usr/bin/env python
 # coding=utf-8
 # Copyright 2018 Google AI, Google Brain and Carnegie Mellon University Authors and the HuggingFace Inc. team.
 # Copyright (c) 2018, NVIDIA CORPORATION.  All rights reserved.

examples/token-classification/run_ner.py

+#!/usr/bin/env python
 # coding=utf-8
 # Copyright 2020 The HuggingFace Team All rights reserved.
 #

examples/token-classification/run_tf_ner.py

+#!/usr/bin/env python
 # coding=utf-8
 # Copyright 2018 The HuggingFace Inc. team.
 #

templates/adding_a_new_example_script/{{cookiecutter.directory_name}}/run_{{cookiecutter.example_shortcut}}.py

+#!/usr/bin/env python
 # coding=utf-8
 # Copyright {{cookiecutter.authors}} and The HuggingFace Inc. team. All rights reserved.
 #