**CoVe**, or **Contextualized Word Vectors**, uses a deep [LSTM](https://paperswithcode.com/method/lstm) encoder from an attentional sequence-to-sequence model trained for machine translation to contextualize word vectors. $\text{CoVe}$ word embeddings are therefore a function of the entire input sequence. These word embeddings can then be used in downstream tasks by concatenating them with $\text{GloVe}$ embeddings:

$$ v = \left[\text{GloVe}\left(x\right), \text{CoVe}\left(x\right)\right]$$

and then feeding these in as features for the task-specific models.

**A2C**, or **Advantage Actor Critic**, is a synchronous version of the [A3C](https://paperswithcode.com/method/a3c) policy gradient method. As an alternative to the asynchronous implementation of A3C, A2C is a synchronous, deterministic implementation that waits for each actor to finish its segment of experience before updating, averaging over all of the actors. This more effectively uses GPUs due to larger batch sizes.

Image Credit: [OpenAI Baselines](https://openai.com/blog/baselines-acktr-a2c/)

Asynchronous Methods for Deep Reinforcement Learning

CoVe

Learned in Translation: Contextualized Word Vectors

**Scaled dot-product attention** is an attention mechanism where the dot products are scaled down by $\sqrt{d_k}$. Formally we have a query $Q$, a key $K$ and a value $V$ and calculate the attention as:

$$ {\text{Attention}}(Q, K, V) = \text{softmax}\left(\frac{QK^{T}}{\sqrt{d_k}}\right)V $$

If we assume that $q$ and $k$ are $d_k$-dimensional vectors whose components are independent random variables with mean $0$ and variance $1$, then their dot product, $q \cdot k = \sum_{i=1}^{d_k} u_iv_i$, has mean $0$ and variance $d_k$.  Since we would prefer these values to have variance $1$, we divide by $\sqrt{d_k}$.

Source	Learned in Translation: Contextualized Word Vectors
Year	2000
Data Source	CC BY-SA - https://paperswithcode.com