add extra optional dependency options

setup.py

@@ -46,6 +46,9 @@
     sdk_path = subprocess.check_output(['xcrun', '--show-sdk-path'])
     os.environ['CFLAGS'] = '-isysroot "{}"'.format(sdk_path.rstrip().decode("utf-8"))
 
+with open('requirements_opt.txt') as f:
+    optional_requirements = f.read().splitlines()
+
 setup(
     name='POT',
     version=__version__,
@@ -71,6 +74,17 @@
     data_files=[],
     setup_requires=["oldest-supported-numpy", "cython>=0.23"],
     install_requires=["numpy>=1.16", "scipy>=1.6"],
+    extras_require={
+        'backend-numpy': [], # in requirements.
+        'backend-jax': ['jax<=0.4.24', 'jaxlib<=0.4.24'],
+        'backend-cupy': [], # should be installed with conda, not pip, or figure out what CUDA version above.
+        'backend-tf': ['tensorflow'],
+        'backend-torch': ['torch_geometric'],
+        'cvxopt': ['cvxopt'], # on it's own to prevent accidental GPL violations
+        'dr': ['scikit-learn', 'pymanopt', 'autograd'],
+        'gnn': ['torch', 'torch_geometric'],
+        'all': optional_requirements
+    },
     python_requires=">=3.6",
     classifiers=[
         'Development Status :: 5 - Production/Stable',