Add sentencepiece to the CI and fix tests
# What does this PR do?

When removing sentencepiece from the dependencies of Transformers, the CI started to skip all tests that required sentencepiece. As a result some failures due to new breaking changes (switch to fast tokenizer by default and the fact we removed `max_len`, deprecated argument of tokenizers) were unnoticed. 

This PR adds back sentenpiece install on all CI checks and fix the resulting failing tests.