Add CLUE Benchmark (11 datasets)
Thanks, @lhoestq! I've addressed the comments. 
Also, I have tried to use `ClassLabel` [when possible](https://github.com/huggingface/nlp/pull/572/files#diff-1026ac7d7b78bf029cb0ebe63162c77dR297). Is there still somewhere else we can use `ClassLabel`?  I believe CI failure is unrelated. Great job! 
Add 11 tasks of [CLUE](https://github.com/CLUEbenchmark/CLUE).