fix for from_pyro with multiple chains

[jeffpollock9]

As per #462 - we should only expand the dimension of the samples if the number of chains is 1. Please note I have not tested this for pyro versions less than 0.3.

For the example in the #462, this now correctly prints:

>>> az.summary(data)
          mean    sd  mc error  hpd 3%  hpd 97%  eff_n  r_hat
alpha    45.46  0.08       0.0   45.30    45.59  142.0   1.00
beta[0]   1.25  0.08       0.0    1.11     1.41  164.0   1.01
beta[1]   2.21  0.09       0.0    2.05     2.40  141.0   1.00
sigma     0.79  0.06       0.0    0.69     0.89   77.0   1.00

when running with 2 chains.

[ahartikainen]

.num_chains is apparently missing from the version used in travis.

I think you need to send posterior or num_chains to function. self is referring to class, it is not defined outside the class.

[ahartikainen]

I think you can skip the function and just duplicate your if-else structure, this way everything should work.

[jeffpollock9]

@ahartikainen thanks. I think num_chains is missing in pyro < 0.3 since they didn't support running multiple chains in parallel at that point - so the untested changes I made were dumb (serves me right for not properly testing). That is at least as far as I can tell looking at e.g. https://github.com/uber/pyro/blob/0.3.0-release/pyro/infer/mcmc/mcmc.py#L103

Hoping it is ok now.

[ahartikainen]

LGTM