Provide mechanism to disable Python NVTX ranges

[jrhemstad]

In C/C++, NVTX ranges can be disabled, effectively eliminating any overhead of the nvtx function calls (albeit this overhead should already be small when a tool is not attached).

The Python NVTX ranges should have a similar feature that makes any annotation a no-op.

[jrhemstad]

@shwina

[sunhs]

I think this is still needed. Apart from overhead of nvtx, we usually do a "warmup" before actually profiling a deep learning model, and thus manually disabling nvtx in the warmup run is necessary.

[shwina]

Does setting the environment variable "NVTX_DISABLE" work?