visit count initialization for the leaf node

[eijoac]

In the search() method of the MCTS class, when a leaf node is reached, after the expansion, action prior probability and leaf state value initialization from the NN, shouldn't we set the leaf node visit count (self.Ns[s]) to 1 instead of 0 (line 101 in the MCTS.py here). This visit count isn't incremented during the backpropagation later, so I think it should be set to 1 at initialization.