Fix division by zero when there are zero-length spans in MismatchedEmbedder.

[dwadden]

Fixes #4612 and adds a unit test to confirm that missing or exotic tokens to not lead to nan gradients in BERT-style embedder.

[matt-gardner]

Thanks for doing this, this looks great! There are a couple of minor things to clean up in the test, and can you add a simple note to the changelog saying something like "Fixed division by zero error when there are zero-length spans in the input to a mismatched embedder."

[matt-gardner]

Can you just make this:

Suggested change

	params = Params(
	{
	"token_embedders": {
	"bert": {
	"type": "pretrained_transformer_mismatched",
	"model_name": "bert-base-uncased",
	}
	}
	}
	)
	token_embedder = BasicTextFieldEmbedder.from_params(vocab=vocab, params=params)
	token_embedder = BasicTextFieldEmbedder({"bert": PretrainedTransformerMismatchedEmbedder("bert-base-uncased")})

No need to use Params here. (be sure to run black on that line, it might be too long, and this might require adding some imports above)

[dwadden]

Changed.

[dwadden]

Changes have been made. There are two transformer parameters that have None gradients. Not sure if this is a bug or not. Let me know.

[matt-gardner]

Thanks @dwadden! I tried pushing a couple of small final fixes to get this to pass CI, but for some reason it didn't let me (maybe because this is from your master branch). I was able to do one of them from the web UI, but I can't add the changelog statement that I mentioned in my comment above. Can you do that? Then I'll merge this.

[dwadden]

OK, I added a message to the changelog. I also got confused and did git push --force on my fork, which clobbered your formatting changes. I ran black to get them back, so now the linter and type checker pass.

[matt-gardner]

Thanks again!