Text backend does not support tallying a subset of variables (raises KeyError) #2560

[Bruschkov]

Example:

with pm.Model() as model:
    a = pm.Gamma('a', mu=10.0, shape=(2,3), sd=2.0)
    b = pm.Gamma('b', mu=10.0, sd=2.0)
    
    trace = pm.sample(trace=[model.a, model.a_log__])
    assert len(trace.varnames) == 2
    
    pm.backends.text.dump('trace.text', trace)
    loaded = pm.backends.text.load('trace.text', model=model)
    
x = loaded[0] #!!! Does not throw key error anymore
x

As described in the issue, the above code would not work in the past and now does.

[fonnesbeck]

LGTM