Support building with headers from nvidia wheels (#2623)

* Support building with headers from nvidia wheels

There are two changes:
1. `import nvidia` returns a namespace package with `__file__` equal to `None`
2. Add the way to force headers from nvidia wheels. Without that envvar, it's practically impossible with CUDA installed system-wide.

I successfully built the package with torch using the following `uv` configuration:
```
[tool.uv.extra-build-dependencies]
"transformer-engine-torch" = [
    "ninja",
    "nvidia-cuda-crt==13.0.88",
    "nvidia-cuda-cccl==13.0.85",
    { requirement = "torch", match-runtime = true },
    { requirement = "pytorch-triton", match-runtime = true },
    { requirement = "nvidia-cusolver", match-runtime = true },
    { requirement = "nvidia-curand", match-runtime = true },
    { requirement = "nvidia-cublas", match-runtime = true },
    { requirement = "nvidia-cusparse", match-runtime = true },
    { requirement = "nvidia-cudnn-cu13", match-runtime = true },
    { requirement = "nvidia-nvtx", match-runtime = true },
    { requirement = "nvidia-cuda-nvrtc", match-runtime = true },
    { requirement = "nvidia-cuda-runtime", match-runtime = true },
]
```
Signed-off-by: Vadim Markovtsev <vadim@poolside.ai>

* Apply suggestion from @ksivaman
Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>
Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>

---------
Signed-off-by: Vadim Markovtsev <vadim@poolside.ai>
Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>
Co-authored-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>

build_tools/utils.py

@@ -228,9 +228,10 @@ def nvcc_path() -> Tuple[str, str]:
 def get_cuda_include_dirs() -> Tuple[str, str]:
     """Returns the CUDA header directory."""
 
+    force_wheels = bool(int(os.getenv("NVTE_BUILD_USE_NVIDIA_WHEELS", "0")))
     # If cuda is installed via toolkit, all necessary headers
     # are bundled inside the top level cuda directory.
-    if cuda_toolkit_include_path() is not None:
+    if not force_wheels and cuda_toolkit_include_path() is not None:
         return [cuda_toolkit_include_path()]
 
     # Use pip wheels to include all headers.
@@ -239,7 +240,10 @@ def get_cuda_include_dirs() -> Tuple[str, str]:
     except ModuleNotFoundError as e:
         raise RuntimeError("CUDA not found.")
 
-    cuda_root = Path(nvidia.__file__).parent
+    if nvidia.__file__ is not None:
+        cuda_root = Path(nvidia.__file__).parent
+    else:
+        cuda_root = Path(nvidia.__path__[0])  # namespace
     return [
         subdir / "include"
         for subdir in cuda_root.iterdir()