[`MT5`] Fix CONFIG_MAPPING issue leading it to load umt5 class
# What does this PR do?
Adresses #24662 and one of our CI test.
The issue stems from the `CONFIG_MAPPING`'s values used as keys to index over the auto mapping. 
There were two ways to fix this, either change our logic or just add a config.
For simplicity added a config. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Can confirm that: 
```python 
from transformers import AutoModelForSeq2SeqLM, AutoTokenizer

model = AutoModelForSeq2SeqLM.from_pretrained('google/mt5-small')
print(type(model))
<class 'transformers.models.mt5.modeling_mt5.MT5ForConditionalGeneration'>
```
is back to normal 😄 