* 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>