Max out span extractor

[MSLars]

Closes #5449.

Changes proposed in this pull request:

• Added MaxPoolingSpanExtractor. This components represents spans via a max-pooling operation.

Before submitting

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
  I received an Error
  E RuntimeError: function '_has_torch_function' already has a docstring
  when I execute

pytest -v --cov allennlp.modules.span_extractors.max_pooling_span_extractor tests/modules/span_extractors/max_pooling_span_extractor_test.py 

to calculate the test coverage.

• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR fixes a bug, I've added a test that will fail without my fix.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

After submitting

• All GitHub Actions jobs for my pull request have passed.
• codecov/patch reports high test coverage (at least 90%).
  You can find this under the "Actions" tab of the pull request once the other checks have finished.

[epwalsh]

Hey @MSLars, thanks for taking this. Are you still having an issue running tests? Also, could you update the CHANGELOG when you get a chance?

[MSLars]

@epwalsh I updates the Changelog. I still have the issue with running

pytest -v --cov  allennlp.modules.span_extractors.max_pooling_span_extractor tests/modules/span_extractors/max_pooling_span_extractor_test.py

which leads to

ERROR tests/modules/span_extractors/max_pooling_span_extractor_test.py - RuntimeError: function '_has_torch_function' already has a docstring

But I seem to have the same problem for each test with import torch.

When I execute

pytest -v --cov  allennlp tests

The same test as before runs without errors. So I guess its an issue with my development configuration and the code should be fine.

[epwalsh]

This looks great, @MSLars, thank you! I just left a few minor comments.

[epwalsh]

LGTM! Thanks @MSLars!