Custom Weibull distribution

Questions
9

This option seems very nice to me, however when running the following code:
import pymc3 as pm

class Translate(pm.distributions.transforms.Transform):
  def __init__(self, v):
    self.v_ = v

  def forward(self, x):
    return x + self.v_

  def forward_val(self, x):
    return x + self.v_

  def backward(self, x):
    return x - self.v_

  def jacobian_det(self, x):
    return 0. * x  # log(1) = 0

  def apply(self, dist):
    return pm.distributions.transforms.TransformedDistribution.dist(dist, self)

with pm.Model() as model:
      l = pm.Gamma('l_pr', alpha=3, beta=3)
      a = pm.Gamma('a', alpha=3, beta=3)
      b = pm.Gamma('b', alpha=3, beta=3)
      shift = Translate(l)
      weib = shift.apply(pm.Weibull('base', alpha=a, beta=b))
      trace = pm.sample(100)

I get the following error:

   Traceback (most recent call last):
  File "C:/Users/xxx/source/xxx/run_scripts/xxx/test.py", line 28, in <module>
    weib = shift.apply(pm.Weibull('base', alpha=a, beta=b))
  File "C:/Users/xxx/source/xxx/run_scripts/xxx/test.py", line 21, in apply
    return pm.distributions.transforms.TransformedDistribution.dist(dist, self)
  File "C:\Users\xxx\AppData\Local\Continuum\anaconda3\envs\xxx\lib\site-packages\pymc3\distributions\distribution.py", line 52, in dist
    dist.__init__(*args, **kwargs)
  File "C:\Users\xxx\AppData\Local\Continuum\anaconda3\envs\xxx\lib\site-packages\pymc3\distributions\transforms.py", line 120, in __init__
    testval = forward(dist.default())
AttributeError: 'TransformedRV' object has no attribute 'default'

Thanks!