Understanding Encoder-Only Transformers: The Foundation of BERT and RAG Retrieval

#ai #machinelearning

Back in 2017, the first transformer architecture introduced two main components:

an encoder
a decoder

These two parts were connected so they could work together.

This original design is known as an encoder–decoder transformer.

Decoders Can Work on Their Own

Over time, researchers realized that the decoder alone was powerful enough for many tasks.

Using only a decoder, models could:

generate text
continue sentences
perform translation and other language tasks

As we discussed in the article on decoder only transformers, these models form the foundation of systems like ChatGPT.

These are called decoder-only transformers.

Encoders Can Also Work Independently

In a similar way, encoder-based models are also very useful on their own.

This idea forms the foundation of models like BERT and many others.

These are called encoder-only transformers.

Building Blocks of Encoder-Only Transformers

Encoder-only transformers use the same core components we explored earlier:

Word embeddings convert words into numbers
Positional encoding keeps track of word order
Self-attention helps establish relationships between words

When these layers are combined, they create a new representation for each token that captures:

meaning
position
relationships with other words

These representations are called context-aware embeddings or contextualized embeddings.

Why Context-Aware Embeddings Are Useful

Context-aware embeddings can help group together:

similar sentences
similar paragraphs
similar documents

This capability is one of the foundations of Retrieval-Augmented Generation (RAG).

RAG works by:

Breaking documents into smaller chunks of text
Using an encoder-only transformer to generate embeddings for each chunk
Comparing embeddings to find the most relevant information

Other Uses of Encoder-Only Transformers

Context-aware embeddings can also be used as inputs for machine learning models.

For example:

neural networks can use them for sentiment classification
logistic regression models can also use them for classification tasks

That wraps up encoder-only transformers.

In the next article, we will explore reinforcement learning in neural networks.

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