Add the number of `model` test failures to slack CI report
# What does this PR do?

We decided to add deepspeed (nightly version) CI job to past CI in #22393. Also `accelerate` is installed with the `main` branch.
This makes the deepspeed CI job have quite a lot of failures most of the time. Sometimes we need to wait DS team for a fix, and sometimes a fix is required from HF.

**Let's add an information regarding `number of model test failures` (i.e. not counting deepspeed CI job's failures) to the Slack CI report**, so we have a number indicating the progress in past CI and make me feel a bit more of fulfillment 🙏 .
The screenshot below shows the issue that motivates this PR

<img width="506" alt="Screenshot 2023-06-12 205537" src="https://github.com/huggingface/transformers/assets/2521628/f6b322e2-a51e-48e9-a438-d1d431470637">
<|||||>_The documentation is not available anymore as the PR was closed or merged._