Threadpool related changes. (#3564)
Threadpool related changes.
Don't create ORT threadpool if openmp is enabled (except for inter op threadpool).
Created a new static function ThreadPool::NumThreads to account for openmp settings and null threadpool ptr.
Log a warning when using SetIntraOpNumThreads when openmp is enabled.
Added a document for ORT devs.
Fix LSTM to use the new threadpool abstractions.
Rename GetNumCpuCores to GetThreadAffinityMasks and move it to the Env class.
Co-authored-by: Tracy Sharpe <tracysh@microsoft.com>
