if 3d model

[haohaiziround]

if 3d model has something wrong for computing grad_logp(logp,x[key]) in line 159 of dhmc alg.

Error as following:

RuntimeError: mul() received an invalid combination of arguments - got (NoneType), but expected one of:
 * (float other)
      didn't match because some of the arguments have invalid types: (!NoneType!)
 * (Variable other)
      didn't match because some of the arguments have invalid types: (!NoneType!)

[haohaiziround]

The model is as following:

x1 ~ N(0, 1)
if x1 > 0:
    x2 ~ N(0, 1)
    observe(N(x2, 1) y)
else:
    x3 ~ N(0, 1)
    observe(N(x3, 1) y)

We have two ways to write the model.
One is

(let [x1 (sample (normal 0 1))
      x2 (sample (normal 0 1))
      x3 (sample (normal 0 1))
      y 1]
  (if (> x1 0)
    (observe (normal x2 1) y)
    (observe (normal x3 1) y))
  (vector x1 x2 x3))

and the other is

(let [x1 (sample (normal 0 1))
      y 10]
  (if (> x1 0)
    (let [x2 (sample (normal 0 1))]
      (observe (normal x2 2) y))
    (let [x3 (sample (normal 0 1))]
      (observe (normal x3 2) y)))
  x1)

The top one is running fine but the bottom one has some issue as mentioned earlier.