Add MakeVector case in get_vector_len, now that it may replace Join.

theano/tensor/basic.py

@@ -2851,6 +2851,8 @@ def get_vector_length(v):
             return join.vec_length(v)
         except ValueError:
             pass
+    if v.owner and isinstance(v.owner.op, theano.tensor.opt.MakeVector):
+        return len(v.owner.inputs)
     if v.owner and isinstance(v.owner.op, Shape):
         return v.owner.inputs[0].type.ndim
     raise ValueError("length not known")