oekenta

interpreting GPT: the logit lens

Hey I'm not finished reading this yet but I noticed something off about what you said.

At the end, the final 1600-dimensional vector is multiplied byW'stranspose to project back into vocab space.

This isn't quite right. They don't multiply by W's transpose at the end. Rather there is a completely new matrix at the end, whose shape is the same as the transpose of W.

You can see this in huggingface's code for GPT2. In the class GPT2LMHeadModel the final matrix multiplication is performed by the matrix called "lm_head", where as the matrix you call W which is used to map 50,257 dimensional vectors into 1600 dimensional space is called "wte" (found in the GPT2Model class). You can see from the code that wte has shape "Vocab size x Embed Size" while lm_head has shape "Embed Size x Vocab size" so lm_head does have the same shape as W transpose but doesn't have the same numbers.

Edit: I could be wrong here, though. Maybe lm_head was set to be equal to wte transpose? I'm looking through the GPT-2 paper but don't see anything like that mentioned.

Thanks for the info.

This was a great read, very informative.