Fix  TF Funnel
# What does this PR do?

This PR fixes Funnel to make it full graph compliant. Even though all the slow/quick tests are passing and got similar results with few experiements, @sgugger I would appreciate that you thoroughly look at the changes in order to be sure no bugs have been introduced.

@LysandreJik feel free to merge if it looks ok for you and if @sgugger approves the last fix on `pooled_hidden`.