emd2 and gw2 mass gradients not centered

[rflamary]

Describe the bug

To Reproduce

just compute the gradient of ot.emd2 wrt the masses

Screenshots

Code sample

Expected behavior

Environment (please complete the following information):

• OS (e.g. MacOS, Windows, Linux):
• Python version:
• How was POT installed (source, pip, conda):
• Build command you used (if compiling from source):
• Only for GPU related bugs:
  • CUDA version:
  • GPU models and configuration:
  • Any other relevant information:

Output of the following code snippet:

import platform; print(platform.platform())
import sys; print("Python", sys.version)
import numpy; print("NumPy", numpy.__version__)
import scipy; print("SciPy", scipy.__version__)
import ot; print("POT", ot.__version__)

Additional context