Change GDS to 1 AIO thread (#6459)
The `numThreads` config option determines how many threads are used to
read from the file. In the CPU case these threads are created via AIO,
in the GDS case they are handled by the GDS library via the cufile.json.
If we were to also create AIO threads it would have a multiplicative
effect. Example 8 AIO threads * 8 GDS threads would be 64 threads
reading from the file when the user really only intended for 8 threads.
Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>
