Python exception in Pool() when pairs_weight is a numpy ndarray

[LachlanStuart]

Problem:

Minimal reproduction:

pool = Pool(
    np.ones((5,1)), 
    np.ones(5), 
    pairs=np.array([[0,1],[2,3]]), 
    pairs_weight=np.array([0.5, 0.5]),
)

Exception:

Traceback (most recent call last):
  File "/home/lachlan/miniconda3/envs/sm38/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3417, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-49-2c10cac71c1a>", line 1, in <module>
    Pool(np.ones((5,1)), np.ones(5), pairs=np.array([[0,1],[2,3]]), pairs_weight=np.array([0.5, 0.5]))
  File "/home/lachlan/miniconda3/envs/sm38/lib/python3.8/site-packages/catboost/core.py", line 628, in __init__
    self._init(data, label, cat_features, text_features, embedding_features, pairs, weight, group_id, group_weight, subgroup_id, pairs_weight, baseline, timestamp, feature_names, thread_count)
  File "/home/lachlan/miniconda3/envs/sm38/lib/python3.8/site-packages/catboost/core.py", line 1171, in _init
    self._init_pool(data, label, cat_features, text_features, embedding_features, pairs, weight, group_id, group_weight, subgroup_id, pairs_weight, baseline, timestamp, feature_names, thread_count)
  File "_catboost.pyx", line 3755, in _catboost._PoolBase._init_pool
  File "_catboost.pyx", line 3803, in _catboost._PoolBase._init_pool
  File "_catboost.pyx", line 3676, in _catboost._PoolBase._init_features_order_layout_pool
  File "_catboost.pyx", line 3311, in _catboost._set_pairs
  File "_catboost.pyx", line 3292, in _catboost._make_pairs_vector
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

It runs correctly if a regular Python list is used for pairs_weight. The bug is clear in the code - it should be if pairs_weight is not None:. I'd make a PR but I have no experience with .pyx code and don't know how to test it after making changes.

catboost version: 1.0.3
Operating System: Ubuntu 20.04
CPU: Intel Core i7
GPU: N/A