Online Variational Autoencoder
The fact that pieces of data arrive one by one does not define the minibatch size. You can have a buffer of size N (with either FIFO or with any other eviction policy that suits the statistical properties needed) and sample minibatches of size M out it every time you want to update the autoencoder.

Depending on the buffer eviction policy and the sampling strategy, this may also help to avoid local autocorrelation.

Note: this answer was originally a comment to the OP's question.