Fixed Masking in HookedTransformer.generate

[tuomaso]

Description

Currently calling HookedTransformer.generate() with a batch of inputs, i.e. a list of two prompts does not correctly mask the padding tokens, causing the batched outputs to be different from the outputs of generating 1-by-1 for the shorter sequences. Current code does not mask attention at all, so I implemented a ~5 line change to mask attention correctly using existing functions. This fix only works when padding_side="left" so I changed the default value for generate to be "left", not sure how to fix this for right padding. After the fix .generate() with a batch gives the same output as generating one by one (with do_sample = False).

Fixes # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Changing the default value/fixing behavior will change some outputs but only ones that were broken already.

Screenshots

Before

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After

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility