Max Pooling Entity Extractor

[MSLars]

Several relation extraction approaches receive span representations through a max-pooling operation
Implementing such approaches (E.g. RE-AGCN in lines 69 and 70) could benefit from a new module.

@SpanExtractor.register("max_pooling")
class MaxPoolingSpanExtractor(SpanExtractorWithSpanWidthEmbedding):

Currently, I can't find such a component or a related feature request and I would offer to prepare a Merge Request if the idea seems useful.

[epwalsh]

Hi @MSLars, once you have a working implementation feel free to open a pull request.