FIX pruning method (potentially exclusive to MIGHT-O)

[PSSF23]

@adam2392 @YuxinB @SUKI-O

Traceback (most recent call last):
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/joblib/_utils.py", line 72, in _call_
    return self.func(**kwargs)
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/joblib/parallel.py", line 598, in _call_
    return [func(*args, **kwargs)
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/joblib/parallel.py", line 598, in <listcomp>
    return [func(*args, **kwargs)
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/treeple/ensemble/_honest_forest.py", line 78, in _parallel_build_trees
    tree._fit(
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/treeple/tree/_honest_tree.py", line 676, in _fit
    self._fit_leaves(X, y, sample_weight=sample_weight_leaves)
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/treeple/tree/_honest_tree.py", line 768, in _fit_leaves
    self._set_leaf_nodes(honest_leaves, y, sample_weight)
  File "/cis/home/iogihar1/miniconda3/envs/sktree/lib/python3.9/site-packages/treeple/tree/_honest_tree.py", line 797, in _set_leaf_nodes
    self.tree_.value[leaf_id][:, yval] += y_weight
IndexError: arrays used as indices must be of integer (or boolean) type

So we found this error on pruned version of MIGHT-O:
https://github.com/neurodata/treeple/blob/main/treeple/tree/_honest_tree.py#L768

@adam2392, you mentioned that these lines might not be necessary? We know that without these lines, pruning doesn't work from @YuxinB's earlier results.