Task: Question Answering
Input (English): Following BIBREF21, we sample $K$ captions for each image when applying REINFORCE: ${\hat{c}}_1 \ldots {\hat{c}}_K$, ${\hat{c}}_k \sim p_{\theta }(c|I)$,

The baseline for each sampled caption is defined as the average reward of the rest samples. 
 Question: What baseline function is used in REINFORCE algorithm?
Output (English): baseline for each sampled caption is defined as the average reward of the rest samples