[Frontend] Adds fwd and bwd to pure_callbacks

[erick-xanadu]

Context:

Description of the Change:

Benefits:

Possible Drawbacks:

Related GitHub Issues: Depends on #742, #735 (The first commit are these two prs squashed, which is what we will see when they are merged. When these are merged, I will just drop and rebase).

[sc-60499]