[D] Why back-propagation is intractable of MoCO key encoder?

In the original paper of [MoCo](https://arxiv.org/abs/1911.05722), it said that:

> Using a queue can make the dictionary large, but it also makes it intractable to update the key encoder by back-propagation (the gradient should propagate to all samples in the queue).

First I thought that the main reason that the bp cannot imply on key encoder is that the queue operation is not differentable. But It seems not true. You can compute the gradient of all samples in the queue, …

dictionary encoder gradient imply machinelearning propagation reason the key thought true