Wait for images before prediction -- time steps LSTM -- callback issue

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Hey everybody!

I have implemented a LSTM deep learning model. I would like to use the trained model to predict autonomous driving commands. I use the LGSVL simulator to interact. During the autonomous driving part, I subscribe to 3 camera sensors and publish in a control command topic. Since they arrive at different rates, I use the ros2 message filters module and its ApproximateTimeSynchronizer function to make a callback.

In the callback, I predict the commands by using the images. Since I use LSTM, I have made my input images as time series sequences according to the timesteps. For example, if the timesteps = 2, the model expects 2 sets of 3 camera images to predict. So that I use both t and t-1 frame images to predict t+1. However, in callback only one set is received.

So my question is how can I wait till I get 2(changes with timesteps defined) sets of images to make a callback? Can I store the images in a list, call them when it reaches appropriate length for prediction?

Any ideas will be appreciated. Thank you.