fix submodule imports by importing functions directly (#6188)

Summary:
fixes two sporadic issues from missing attributes:

- breaking circular imports
- submodule not being imported explicitly

Reviewed By: ehhuang

Differential Revision: D37071652

fbshipit-source-id: 0680f098384b0fd21076339750e9d1a96186ede3
Co-authored-by: Edward Wang (EcoF) <hack@fb.com>

torchvision/extension.py

@@ -47,10 +47,10 @@ def _check_cuda_version():
     """
     if not _HAS_OPS:
         return -1
-    import torch
+    from torch.version import cuda as torch_version_cuda
 
     _version = torch.ops.torchvision._cuda_version()
-    if _version != -1 and torch.version.cuda is not None:
+    if _version != -1 and torch_version_cuda is not None:
         tv_version = str(_version)
         if int(tv_version) < 10000:
             tv_major = int(tv_version[0])
@@ -58,8 +58,7 @@ def _check_cuda_version():
         else:
             tv_major = int(tv_version[0:2])
             tv_minor = int(tv_version[3])
-        t_version = torch.version.cuda
-        t_version = t_version.split(".")
+        t_version = torch_version_cuda.split(".")
         t_major = int(t_version[0])
         t_minor = int(t_version[1])
         if t_major != tv_major or t_minor != tv_minor: