add amazon reviews
@patrickvonplaten Yeah this is adapted from tfds so a lot is just how they wrote the code. Addressed your comments and also simplified the weird `AmazonUSReviewsConfig` definition. Will merge once tests pass. Thanks for checking this one :) 
Looks good to me 

Just one question : is there a particular reason to use `names=["Y", "N"]` in this order ? Usually the positive label is at index 1 and the negative one at index 0 for binary classification > is there a particular reason to use `names=["Y", "N"]` in this order ? Usually the positive label is at index 1 and the negative one at index 0 for binary classification

Hmm that's a good point. I'll submit a quick fix.


Adds the Amazon US Reviews dataset as requested in #353. Converted from [TensorFlow Datasets](https://www.tensorflow.org/datasets/catalog/amazon_us_reviews). cc @clmnt @sshleifer 