Add -e flag to some GH workflow yml files
# What does this PR do?

Current 2 GitHub actions workflow yml use `pip install [.dev]`. This installs `transformers` in `/home/runner/venv/lib/python3.6/site-packages`, and this is cached. In future job runs, the cache is restored, and that `transformers` version is used - instead of the latest commit, i.e. we want to use `/home/runner/work/transformers/transformers/src`.

Without this PR, I have trouble after updating `add_new_model.py` (to change test model folders from `tests/` to `tests/models/`) because the `add_new_model.py` from `/home/runner/venv/lib/python3.6/site-packages` would be used, which would put the test template models under `tests/`

This PR makes sure the latest `transformers` is used by using `pip install -e [.dev]`, and builds a new cache with it.

- Add -e flag to some GH workflow yml files
- change cache key in order to make the change effective
- add a check on `transformers` location
_The documentation is not available anymore as the PR was closed or merged._<|||||>The added check will produce something like (if the transformers is not from the expected location)

<img width="777" alt="Screenshot 2022-04-27 131320" src="https://user-images.githubusercontent.com/2521628/165506295-901fd17d-17f6-4fb7-8eec-f96b87d63b03.png">

<|||||>after the new cache is built the first time and job completes (pass the check I added too), the next run when we have 

```
Cache restored from key: v3-tests_model_like-ce386d6c28d7afcca58dc875de2ef1b7477e8246a0bfdb6ff4de0eb222eafef2
```
the check failed with

```
transformers is from  but it shoud be from /home/runner/work/transformers/transformers/src.
A fix is required. Stop testing.
```
which means `pip show transformers` gives empty location!

I will try to make it work - I really like to have this test.
~~(But things should work now if we just remove this test)~~

<|||||>Confirmed this (with `-e`) currently not working for the subsequentially run (i.e. cache loaded).

I still think there might be some workaround, let me try. Set to draft for now

### currently error
```
Traceback (most recent call last):
  File "/home/runner/venv/bin/transformers-cli", line 33, in <module>
    sys.exit(load_entry_point('transformers', 'console_scripts', 'transformers-cli')())
  File "/home/runner/venv/bin/transformers-cli", line [22](https://github.com/huggingface/transformers/runs/6197315688?check_suite_focus=true#step:7:22), in importlib_load_entry_point
    for entry_point in distribution(dist_name).entry_points
  File "/home/runner/venv/lib/python3.6/site-packages/importlib_metadata/__init__.py", line 815, in distribution
    return Distribution.from_name(distribution_name)
  File "/home/runner/venv/lib/python3.6/site-packages/importlib_metadata/__init__.py", line 430, in from_name
    raise PackageNotFoundError(name)
importlib_metadata.PackageNotFoundError: No package metadata was found for transformers
Error: Process completed with exit code 1.
```