Extend the `%cd` syntax with commenting (`Block_comment` with string interpolation)

[lukstafi]

For example, the comment breaks the flow here:

let sgd_one ?(lr = 0.001) ?(momentum = 0.0) ?(weight_decay = 0.0) ?(nesterov = false) p =
  if not @@ is_param p then raise @@ Tensor.Session_error ("Train.sgd_one: not a parameter", Some p);
  let pg = NTDSL.term ~label:(p.value.label ^ " sgd delta") () in
  let b = NTDSL.term ~label:(p.value.label ^ " sgd momentum") () in
  Assignments.Block_comment
    ( desc_label_suffix p.value.label ^ " param sgd step",
      [%cd
        pg =: p.grad + (!.weight_decay *. p);
        if Float.(momentum > 0.0) then (
          b =: (!.momentum *. b) + pg;
          if nesterov then pg =+ !.momentum *. b else pg =: b);
        p =- !.lr *. pg] )

Would be nicer with something like (syntax to be decided):

let sgd_one ?(lr = 0.001) ?(momentum = 0.0) ?(weight_decay = 0.0) ?(nesterov = false) p =
  if not @@ is_param p then raise @@ Tensor.Session_error ("Train.sgd_one: not a parameter", Some p);
  let pg = NTDSL.term ~label:(p.value.label ^ " sgd delta") () in
  let b = NTDSL.term ~label:(p.value.label ^ " sgd momentum") () in
  [%cd
    # p " param sgd step";
    pg =: p.grad + (!.weight_decay *. p);
    if Float.(momentum > 0.0) then (
      b =: (!.momentum *. b) + pg;
      if nesterov then pg =+ !.momentum *. b else pg =: b);
    p =- !.lr *. pg]