[TF 2.2 compat]  use tf.VariableAggregation.ONLY_FIRST_REPLICA
This PR updates the TF gradient accumulator optimization to make it compliant with the new TF 2.2.
Good to be merged for me :) We should update the `setup.py` in same time to do not limit anymore to TF 2.1. Should it be done in this PR? /cc @julien-c @LysandreJik <|||||>I think you can cc @LysandreJik @julien-c <|||||>Yes can you do it in the same PR? and it'll be ready to merge.<|||||>Nice! Unfortunately there are other issues with TF2.2, hence why the tests fail and why the version was pinned. I'm patching it now.<|||||>Yep I think @mfuntowicz is working on it as it is related to the Tokenizer. At least we have identified why it is not working :)