minor bug fix

[raymond-yuan]

• fixed entropy sign
• changed entropy implementation for numeric stability

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[raymond-yuan]

I signed it!

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CLAs look good, thanks!

[qlzh727]

Not sure who is the owner of this code, @karmel and @fchollet.

[qlzh727]

Is there any reason that the entropy is negative here?

[qlzh727]

I think "policy_loss -= 0.01 * entropy" might be correct, overwriting the policy_loss does not make sense.

[MarkDaoust]

You are totally correct. I fixed this before merging.

[MarkDaoust]

@qlzh727 I think that negative is there because of the -= later on.
I removed the double negative.

But that didn't look right. This "entropy regularization" is supposed to discourage premature convergence, so it should encourage a high entropy not a low entropy.

There seems to be a lot of confusion around this point in other tutorials on the subject, because of the way the paper is written.

I think this (subtracting the entropy) is right. I'm merging as is.

@raymond-yuan LMK if you disagree.