CLI script to gather environment info (#2699)

* add "info" command to CLI

As a convenience, add the info directive to CLI. Running `python transformers-cli info` will return a string containing the transformers version, platform, python version, PT/TF version and GPU support

* Swap f-strings for .format

Still supporting 3.5 so can't use f-strings (sad face)

* Add reference in issue to CLI

* Add the expected fields to issue template

This way, people can still add the information manually if they want. (Though I fear they'll just ignore it.)

* Remove heading from output

* black-ify

* order of imports

Should ensure isort test passes

* use is_X_available over import..pass

* style

* fix copy-paste bug

* Rename command info -> env

Also adds the command to CONTRIBUTING.md in "Did you find a bug" section

CONTRIBUTING.md

@@ -41,14 +41,10 @@ Did not find it? :( So we can act quickly on it, please follow these steps:
   less than 30s;
 * Provide the *full* traceback if an exception is raised.
 
-To get the OS and software versions, execute the following code and copy-paste
-the output:
+To get the OS and software versions automatically, you can run the following command:
 
-```
-import platform; print("Platform", platform.platform())
-import sys; print("Python", sys.version)
-import torch; print("PyTorch", torch.__version__)
-import tensorflow; print("Tensorflow", tensorflow.__version__)
+```bash
+python transformers-cli env
 ```
 
 ### Do you want to implement a new model?