Unroll, then walk the same graph backwards
∂ℒ/∂θ = ∑
k
∂ℒ
k
/∂θ
Next
Play
Reset
① the graph
one unroll, two directions
② one step back
∂ℓ
k
/∂h
k
+ g
k+1
J
k+1
③ the chain
④ onto θ
sequence length
phase
numbers