add torch as torch_scatter requirements

setup.py

@@ -107,7 +107,8 @@ def get_extensions():
     return extensions
 
 
-install_requires = []
+install_requires = ["torch>=1.8.0"]
+extra_index_url = ["https://download.pytorch.org/whl/"]
 
 test_requires = [
     'pytest',
@@ -130,6 +131,7 @@ def get_extensions():
     keywords=['pytorch', 'scatter', 'segment', 'gather'],
     python_requires='>=3.8',
     install_requires=install_requires,
+    extra_index_url=extra_index_url,
     extras_require={
         'test': test_requires,
     },