update setup

setup.py

@@ -1,5 +1,8 @@
 from setuptools import setup
 
+with open("README.md", "r", encoding="utf-8") as fh:
+    long_description = fh.read()
+
 setup(
     name="torch_struct",
     version="0.5",
@@ -9,9 +12,12 @@
         "torch_struct",
         "torch_struct.semirings",
     ],
+    long_description=long_description,
     package_data={"torch_struct": []},
+    long_description_content_type="text/markdown",
     url="https://github.com/harvardnlp/pytorch_struct",
     install_requires=["torch"],
     setup_requires=["pytest-runner"],
     tests_require=["pytest"],
+    python_requires='>=3.6',
 )

torch_struct/semirings/checkpoint.py

@@ -1,8 +1,10 @@
 import torch
 
+has_genbmm = False
 try:
     import genbmm
     from genbmm import BandedMatrix
+    has_genbmm = True
 except ImportError:
     pass
 
@@ -52,7 +54,7 @@ def backward(ctx, grad_output):
     class _CheckpointSemiring(cls):
         @staticmethod
         def matmul(a, b):
-            if isinstance(a, genbmm.BandedMatrix):
+            if has_genbmm and isinstance(a, genbmm.BandedMatrix):
                 lu = a.lu + b.lu
                 ld = a.ld + b.ld
                 c = _CheckBand.apply(a.data, a.lu, a.ld, b.data, b.lu, b.ld)