trajectory function use target_trajectory as input in loss_and_gradient

Hi,

I'm trying to understand the logic behind the various functions in ml.train_utils.

in the definition of loss_and_gradient the returned _loss function uses the target trajectory as part of the input of the trajectory_fn. I cannot make sense of it. The description of the function states that the trajectory_fn should accepts `params` and `initial_velocity`, which make sense to me, but then I don't understand why would we want to use the target_trajectory as the initial_velocity (which presumably doesn't have the same shape as initial_velocity since one is a trajectory and the other the velocity at a single time step).

I'm pretty much stuck here because I now don't know how to use this in the context of defining a train_step function etc.

Best regards,

Gwen