Fix disappearing probability mass #2
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Closes #1. Not sure if this is the best way to deal with the problem, but it seems to cover a few cases - let me know if you have a better solution.
So to provide an example on the issue, the update relies on
qa_probs * (u.float() - b)andqa_probs * (b - l.float()), but ifbhappens to contain any ints (e.g. in a terminal state where all probability mass is concentrated in one location), then both of these parts of the update turn intoqa_probs * 0and hence the network tries matching a vector of 0s with its softmax output, which will obviously cause problems. Due to the nature of this edge case I believe it has a worse effect on environments with more terminal transitions.