Document all available build-time and run-time environment variables

[leofang]

We should also rename PARALLEL_LEVEL to, e.g., CUDA_PYTHON_PARALLEL_LEVEL

cuda-python/setup.py

Line 31 in e1e3325

nthreads = int(os.environ.get("PARALLEL_LEVEL", "0") or "0")

[leofang]

related: #133 (CUDA_PYTHON_CUDA_PER_THREAD_DEFAULT_STREAM)

[ksimpson-work]

I am naively addressing the list of environment variables I have found referenced in the codebase by grepping os.environ.get(. Is there a list or source of truth I can look at which documents other variables that we want to document which aren't explicitely queried in our .py source files? The list I currently have is short:

CUDA_PATH / CUDA_HOME
CUDA_PYTHON_PARALLEL_LEVEL
CUDA_PYTHON_PARSER_CACHING
CUDA_PYTHON_DOMAIN

[leofang]

• CUDA_PATH/CUDA_HOME: build-time only
• CUDA_PYTHON_PARALLEL_LEVEL: build-time only
• CUDA_PYTHON_PARSER_CACHING: build-time only
• CUDA_PYTHON_CUDA_PER_THREAD_DEFAULT_STREAM: run-time only