Thanks for the answer. The t1* tensors are possibly differently shaped. Actually (t11, t12) is an observation, the list is a trajectory of observations, and what I try to do is to build a new observation by picking a different point in the trajectory for each element of the batch because only one observation is interesting in each trajectory. Then I can do a single forward pass in my model.
I guess it will be more efficient to do one forward pass per element of the trajectory, and then pick the relevant results instead of what I am trying to do here, I will try that.