Bugs in Build _ext module

[cuge1995]

pointnet2/_ext-src/src/group_points.cpp:56:40: error: ‘AT_CHECK’ was not declared in this scope
     AT_CHECK(false, "CPU not supported");
                                        ^
error: command 'gcc' failed with exit status 1

[cuge1995]

solved by pip install torchvision==0.5 and pip install torch==1.4

[sidml]

If you are using torch>1.5, then you need to replace AT_CHECK with AT_ASSERT.
See facebookresearch/votenet#97