QueryByCommittee 'kl_divergence' method error

[tsiakmaki]

Hi.
thank you for your contribution.
When testing the QueryByCommittee and the 'kl_divergence' method, i get the above error:

mytest.py", line 90, in run
    ask_id = qs.make_query()
  File ".../python3.7/site-packages/libact/query_strategies/query_by_committee.py", line 207, in make_query
    np.where(np.isclose(avg_kl, np.max(avg_kl)))[0])
  File "mtrand.pyx", line 902, in numpy.random.mtrand.RandomState.choice
ValueError: 'a' cannot be empty unless no samples are taken

In my case, it seems that avg_kl
https://github.com/ntucllab/libact/blob/master/libact/query_strategies/query_by_committee.py#L205
contains NaN, and as such the max is nan.
But also there are cases where all values are Nan, and i get the

ValueError: zero-size array to reduction operation maximum which has no identity

Just a notice, in case it makes sense to anyone else.

[yangarbiter]

I think the nan should come from here, if predict_proba from the classifier outputs a 0 probability, the kldivergence will have a nan value

libact/libact/query_strategies/query_by_committee.py

Line 156 in c3babc8

kl = np.sum(proba * np.log(proba / consensus), axis=2)