ot.dist crashes for hamming distance without weights

[nklkhlr]

Bug Description

When calling ot.dist with metric set to hamming an AttributeError is thrown from within scipy.cdist

To Reproduce

(tested using python 3.10, ot version 0.8.2 and scipy version 1.9.1 as well as 1.8.0):

import ot
import numpy as np

x = np.random.randint(0, 2, 20).reshape(4, 5)
y = np.random.randint(0, 2, 20).reshape(4, 5)

ot.dist(x, y, metric="hamming")

Environment

• OS: Ubuntu 22.04
• Python version: 3.10
• Install: conda

Traceback:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "miniconda3/envs/mtda/lib/python3.10/site-packages/ot/utils.py", line 237, in dist
    return cdist(x1, x2, metric=metric, w=w)
  File "miniconda3/envs/mtda/lib/python3.10/site-packages/scipy/spatial/distance.py", line 2929, in cdist
    return cdist_fn(XA, XB, out=out, **kwargs)
  File "miniconda3/envs/mtda/lib/python3.10/site-packages/scipy/spatial/distance.py", line 1673, in __call__
    XA, XB, typ, kwargs = _validate_cdist_input(
  File "miniconda3/envs/mtda/lib/python3.10/site-packages/scipy/spatial/distance.py", line 205, in _validate_cdist_input
    kwargs = _validate_kwargs((XA, XB), mA + mB, n, **kwargs)
  File "miniconda3/envs/mtda/lib/python3.10/site-packages/scipy/spatial/distance.py", line 225, in _validate_hamming_kwargs
    if w.ndim != 1 or w.shape[0] != n:
AttributeError: 'NoneType' object has no attribute 'ndim'

The same error is raised when scipy.cdist is called with w=None

from scipy.spatial.distance import cdist

cdist(x, y, metric="hamming", w=None)

Hence it seems like not passing w here https://github.com/PythonOT/POT/blob/951209ac3f01c86b35d3beff4679ce47e47c0872/ot/utils.py#L237

when w is None should resolve the bug.