model.py: fix issue with length 1 shared variables

 - this patch fixes an issue highlighted in pymc-devs#3122
   where imputation of a single missing observation fails.
   It implements the suggestion from the theano error message
   to manual force a TensorType change in cases where the
   variable has a length of one.

Author: mattpitkin

pymc3/model.py

@@ -437,6 +437,9 @@ def __init__(self, cost, grad_vars, extra_vars=None, dtype=None,
         self._extra_vars_shared = {}
         for var in extra_vars:
             shared = theano.shared(var.tag.test_value, var.name + '_shared__')
+            if isinstance(var.tag.test_value, np.ndarray):
+                if len(var.tag.test_value) == 1:
+                    shared.type = theano.tensor.TensorType(var.dtype, (True,))
             self._extra_vars_shared[var.name] = shared
             givens.append((var, shared))