Do not set ccbin in torch 1.7.x

Summary:
PyTorch versions 1.7.0 and 1.7.1 are between https://github.com/pytorch/pytorch/pull/43931 and https://github.com/pytorch/pytorch/pull/47404. In this gap, PyTorch always copies CC to nvcc_args, like PyTorch3D does. Newer nvcc versions are not happy with `-ccbin` being specified twice, even if it is specified twice the same. We update PyTorch3D so that it doesn't supply `-ccbin` in these cases.

Also tweak the detection of the current ccbin so that it is aware that `-ccbin foo` and `-ccbin=foo` are equivalent.

Reviewed By: theschnitz

Differential Revision: D25825468

fbshipit-source-id: b04e7718cf01820649518eedda99c399c732e8af

setup.py

@@ -5,12 +5,31 @@ import glob
 import os
 import runpy
 import warnings
+from typing import List, Optional
 
 import torch
 from setuptools import find_packages, setup
 from torch.utils.cpp_extension import CUDA_HOME, CppExtension, CUDAExtension
 
 
+def get_existing_ccbin(nvcc_args: List[str]) -> Optional[str]:
+    """
+    Given a list of nvcc arguments, return the compiler if specified.
+
+    Note from CUDA doc: Single value options and list options must have
+    arguments, which must follow the name of the option itself by either
+    one of more spaces or an equals character.
+    """
+    last_arg = None
+    for arg in reversed(nvcc_args):
+        if arg == "-ccbin":
+            return last_arg
+        if arg.startswith("-ccbin="):
+            return arg[7:]
+        last_arg = arg
+    return None
+
+
 def get_extensions():
     this_dir = os.path.dirname(os.path.abspath(__file__))
     extensions_dir = os.path.join(this_dir, "pytorch3d", "csrc")
@@ -61,13 +80,18 @@ def get_extensions():
 
         # This is needed for pytorch 1.6 and earlier. See e.g.
         # https://github.com/facebookresearch/pytorch3d/issues/436
+        # It is harmless after https://github.com/pytorch/pytorch/pull/47404 .
+        # But it can be problematic in torch 1.7.0 and 1.7.1
+        if torch.__version__[:4] != "1.7.":
             CC = os.environ.get("CC", None)
             if CC is not None:
+                existing_CC = get_existing_ccbin(nvcc_args)
+                if existing_CC is None:
                     CC_arg = "-ccbin={}".format(CC)
-            if CC_arg not in nvcc_args:
-                if any(arg.startswith("-ccbin") for arg in nvcc_args):
-                    raise ValueError("Inconsistent ccbins")
                     nvcc_args.append(CC_arg)
+                elif existing_CC != CC:
+                    msg = f"Inconsistent ccbins: {CC} and {existing_CC}"
+                    raise ValueError(msg)
 
         extra_compile_args["nvcc"] = nvcc_args