Adapter Methods

On this page, we present all adapter methods currently integrated into the adapter-transformers library. A tabulary overview of adapter methods is provided here Additionally, options to combine multiple adapter methods in a single setup are presented on the next page.

Bottleneck Adapters

Configuration class: AdapterConfig

Bottleneck adapters introduce bottleneck feed-forward layers in each layer of a Transformer model. Generally, these adapter layers consist of a down-projection matrix \(W_{down}\) that projects the layer hidden states into a lower dimension \(d_{bottleneck}\), a non-linearity \(f\), an up-projection \(W_{up}\) that projects back into the original hidden layer dimension and a residual connection \(r\):

\[ h \leftarrow W_{up} \cdot f(W_{down} \cdot h) + r \]

Depending on the concrete adapter configuration, these layers can be introduced at different locations within a Transformer block. Further, residual connections, layer norms, activation functions and bottleneck sizes etc. can be configured.

The most important configuration hyperparameter to be highlighted here is the bottleneck dimension \(d_{bottleneck}\). In adapter-transformers, this bottleneck dimension is specified indirectly via the reduction_factor attribute of a configuration. This reduction_factor defines the ratio between a model’s layer hidden dimension and the bottleneck dimension, i.e.:

\[ \text{reduction_factor} = \frac{d_{hidden}}{d_{bottleneck}} \]

A visualization of further configuration options related to the adapter structure is given in the figure below. For more details, refer to the documentation of AdapterConfig.

Adapter architectures

Visualization of possible adapter configurations with corresponding dictionary keys.

adapter-transformers comes with pre-defined configurations for some bottleneck adapter architectures proposed in literature:


from transformers.adapters import AdapterConfig

config = AdapterConfig(mh_adapter=True, output_adapter=True, reduction_factor=16, non_linearity="relu")
model.add_adapter("bottleneck_adapter", config=config)


Language Adapters - Invertible Adapters

Configuration class: PfeifferInvConfig, HoulsbyInvConfig

The MAD-X setup (Pfeiffer et al., 2020) proposes language adapters to learn language-specific transformations. After being trained on a language modeling task, a language adapter can be stacked before a task adapter for training on a downstream task. To perform zero-shot cross-lingual transfer, one language adapter can simply be replaced by another.

In terms of architecture, language adapters are largely similar to regular bottleneck adapters, except for an additional invertible adapter layer after the LM embedding layer. Embedding outputs are passed through this invertible adapter in the forward direction before entering the first Transformer layer and in the inverse direction after leaving the last Transformer layer. Invertible adapter architectures are further detailed in Pfeiffer et al. (2020) and can be configured via the inv_adapter attribute of the AdapterConfig class.


from transformers.adapters import PfeifferInvConfig

config = PfeifferInvConfig()
model.add_adapter("lang_adapter", config=config)



V1.x of adapter-transformers made a distinction between task adapters (without invertible adapters) and language adapters (with invertible adapters) with the help of the AdapterType enumeration. This distinction was dropped with v2.x.

Prefix Tuning

Configuration class: PrefixTuningConfig

Illustration of Prefix Tuning.

Illustration of the Prefix Tuning method within one Transformer layer. Trained components are colored in shades of magenta.

Prefix Tuning (Li and Liang, 2021) introduces new parameters in the multi-head attention blocks in each Transformer layer. More, specifically, it prepends trainable prefix vectors \(P^K\) and \(P^V\) to the keys and values of the attention head input, each of a configurable prefix length \(l\) (prefix_length attribute):

\[ head_i = \text{Attention}(Q W_i^Q, [P_i^K, K W_i^K], [P_i^V, V W_i^V]) \]

Following the original authors, the prefix vectors in \(P^K\) and \(P^V\) are note optimized directly, but reparameterized via a bottleneck MLP. This behavior is controlled via the flat attribute of the configuration. Using PrefixTuningConfig(flat=True) will create prefix tuning vectors that are optimized without reparameterization.


from transformers.adapters import PrefixTuningConfig

config = PrefixTuningConfig(flat=False, prefix_length=30)
model.add_adapter("prefix_tuning", config=config)

As reparameterization using the bottleneck MLP is not necessary for performing inference on an already trained Prefix Tuning module, adapter-transformers includes a function to “eject” a reparameterized Prefix Tuning into a flat one:


This will only retain the necessary parameters and reduces the size of the trained Prefix Tuning.



Configuration class: CompacterConfig, CompacterPlusPlusConfig

Illustration of Compacter.

Illustration of the Compacter method within one Transformer layer. Trained components are colored in shades of magenta.

The Compacter architecture proposed by Mahabadi et al., 2021 is similar to the bottleneck adapter architecture. It only exchanges the linear down- and up-projection with a PHM layer. Unlike the linear layer, the PHM layer constructs its weight matrix from two smaller matrices, which reduces the number of parameters. These matrices can be factorized and shared between all adapter layers. You can exchange the down- and up-projection layers from any of the bottleneck adapters described in the previous section for a PHM layer by specifying use_phm=True in the config.

The PHM layer has the following additional properties: phm_dim, shared_phm_rule, factorized_phm_rule, learn_phm, factorized_phm_W, shared_W_phm, phm_c_init, phm_init_range, hypercomplex_nonlinearity

For more information check out the AdapterConfig class.

To add a Compacter to your model you can use the predefined configs:

from transformers.adapters import CompacterConfig

config = CompacterConfig()
model.add_adapter("dummy", config=config)



Configuration class: LoRAConfig

Illustration of LoRA.

Illustration of the LoRA method within one Transformer layer. Trained components are colored in shades of magenta.

Low-Rank Adaptation (LoRA) is an efficient fine-tuning technique proposed by Hu et al. (2021). LoRA injects trainable low-rank decomposition matrices into the layers of a pre-trained model. For any model layer expressed as a matrix multiplication of the form \(h = W_0 x\), it therefore performs a reparameterization, such that:

\[ h = W_0 x + \frac{\alpha}{r} B A x \]

Here, \(A \in \mathbb{R}^{r\times k}\) and \(B \in \mathbb{R}^{d\times r}\) are the decomposition matrices and \(r\), the low-dimensional rank of the decomposition, is the most important hyperparameter.

While, in principle, this reparameterization can be applied to any weights matrix in a model, the original paper only adapts the attention weights of the Transformer self-attention sub-layer with LoRA. adapter-transformers additionally allows injecting LoRA into the dense feed-forward layers in the intermediate and output components of a Transformer block. You can configure the locations where LoRA weights should be injected using the attributes in the LoRAConfig class.


from transformers.adapters import LoRAConfig

config = LoRAConfig(r=8, alpha=16)
model.add_adapter("lora_adapter", config=config)

In the design of LoRA, Hu et al. (2021) also pay special attention to keeping the inference latency overhead compared to full fine-tuning at a minimum. To accomplish this, the LoRA reparameterization can be merged with the original pre-trained weights of a model for inference. Thus, the adapted weights are directly used in every forward pass without passing activations through an additional module. In adapter-transformers, this can be realized using the built-in merge_adapter() method:


To continue training on this LoRA adapter or to deactivate it entirely, the merged weights first have to be reset again:




Configuration class: IA3Config

Illustration of (IA)^3.

Illustration of the (IA)^3 method within one Transformer layer. Trained components are colored in shades of magenta.

Infused Adapter by Inhibiting and Amplifying Inner Activations ((IA)^3) is an efficient fine-tuning method proposed within the T-Few fine-tuning approach by Liu et al. (2022). (IA)^3 introduces trainable vectors \(l_W\) into different components of a Transformer model which perform element-wise rescaling of inner model activations. For any model layer expressed as a matrix multiplication of the form \(h = W x\), it therefore performs an element-wise multiplication with \(l_W\), such that:

\[ h = l_W \odot W x \]

Here, \(\odot\) denotes element-wise multiplication where the entries of \(l_W\) are broadcasted to the shape of \(W\).


from transformers.adapters import IA3Config

config = IA3Config()
model.add_adapter("ia3_adapter", config=config)

The implementation of (IA)^3, as well as the IA3Config class, are derived from the implementation of LoRA, with a few main modifications. First, (IA)^3 uses multiplicative composition of weights instead of additive composition as in LoRA. Second, the added weights are not further decomposed into low-rank matrices. Both of these modifications are controlled via the composition_mode configuration attribute by setting composition_mode="scale". Additionally, as the added weights are already of rank 1, r=1 is set.

Beyond that, both methods share the same configuration attributes that allow you to specify in which Transformer components rescaling vectors will be injected. Following the original implementation, IA3Config adds rescaling vectors to the self-attention weights (selfattn_lora=True) and the final feed-forward layer (output_lora=True). Further, you can modify which matrices of the attention mechanism to rescale by leveraging the attn_matrices attribute. By default, (IA)^3 injects weights into the key (‘k’) and value (‘v’) matrices, but not in the query (‘q’) matrix.

Finally, similar to LoRA, (IA)^3 also allows merging the injected parameters with the original weight matrices of the Transformer model. E.g.:

# Merge (IA)^3 adapter

# Reset merged weights