discovered that  . I think finally I will use this method, ie b[a_idxes], since:

actually, I only need to form a_idxes once, but there are a zillion different b tensors, all of which can be indexed using the exact same a_idxes

using b[a_idxes] notation allows update of the original tensors, at the end of the batch

=> going to rename to ‘[Closed]’