Patch int8PrepareBias to handle nullptr

[jelmervdl]

Description

This fixes base student models not working with the wasm version of bergamot-translator.

int8PrepareBias is being called from both PrepareBiasForBNodeOp and PrepareFakeBiasForBNodeOp. These are two different intgemm calls (callbacks differ) but they map to the same WASM surrogate function.

I'm using the fact that one of those calls has bias_input = nullptr as a way to differentiate between the two and call the correct intgemm function + callback pair in its implementation.

This will also need to be fixed in Mozilla's tree, @abhi-agg.

While on the topic of Mozilla's implementation, should the bounds check in Mozilla's implementation also pick up nullptrs?

See issue #81 for a more detailed explanation.

List of changes:

• Fix int8PrepareBias fallback to call intgemm::Int8Shift::PrepareBias with the correct callback.
• Also fixes a nullptr dereference that would be crash-worthy were it to run natively.

How to test

See #81 how I tested this. To test whether the fix itself works, easiest way would be:

1. Check out https://github.com/jelmervdl/firefox-translations
2. Change marian-dev submodule inside the bergamot-translator submodule to point to this branch
3. Build the wasm module. There is a build-docker-wasm.sh script in the repository. You can use that, but you have to make sure it doesn't re-checkout the dependencies. There is a cmake flag, GIT_SUBMODULE, that stops cmake from doing that.
4. After build-docker-wasm.sh (which also copies the .wasm and .js file) you need to go into the bergamot-translation-worker.js file and force it to always use the fallback implementation, not the optimised one in Firefox itself.
5. Add the entries.reverse() as mentioned here: TranslateLocally _base_ models do not work jelmervdl/translatelocally-web-ext#14.
6. Go to German wikipedia for example, and translate it to English.

Checklist

• I have tested the code manually
• I have run regression tests
• I have read and followed CONTRIBUTING.md
• I have updated CHANGELOG.md

[XapaJIaMnu]

Just a minor suggestion.

[XapaJIaMnu]

Minor, stylistic. I prefer if (input_bias), and I have used this style of nullptr checking elsewhere.