[ASR] hybrid models ignore OMP_NUM_THREADS/torch.set_num_threads() setting
Very odd. 

Hybrid code doesn't explicitly set num threads anywhere. Transcribe() is also never called during training. 

What about num workers on data loader, that's the only place I can think of that spawns workers threads.  There are quite a few locations, where `num_threads()` is updated (not `torch` explicitly, but `numba`), and many of them check `multiprocessing.cpu_count()`, but the real source of the issue here is Numba. More precisely In the case of fastconformer_hybrid it is this section
https://github.com/NVIDIA/NeMo/blob/76a712a323c70e5701a730048627509a11336424/nemo/collections/asr/parts/numba/rnnt_loss/utils/cuda_utils/gpu_rnnt.py#L85-L89

Here the call becomes `numba.set_num_threads(1)`, which resets the torch num_threads. It turns out that Numba resets the torch num_thread even when just reading the Numba num_threads. This can easily be checked with the following snippet:
``` python
$ srun --mem=48G -c32 --container-image=./nemo_head.sqfs --pty bash
root@x:/workspace/nemo# nproc
32
root@x:/workspace/nemo# export OMP_NUM_THREADS=8
root@x:/workspace/nemo# nproc
8
root@x:/workspace/nemo# python3
Python 3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import multiprocessing as mp, numba, torch
>>> mp.cpu_count()
256

>>> torch.get_num_threads()
8

>>> print(torch.__config__.parallel_info())
ATen/Parallel:
        at::get_num_threads() : 8
        at::get_num_interop_threads() : 128
OpenMP 201511 (a.k.a. OpenMP 4.5)
        omp_get_max_threads() : 8
Intel(R) oneAPI Math Kernel Library Version 2021.1-Product Build 20201104 for Intel(R) 64 architecture applications
        mkl_get_max_threads() : 8
Intel(R) MKL-DNN v2.7.3 (Git Hash N/A)
std::thread::hardware_concurrency() : 256
Environment variables:
        OMP_NUM_THREADS : 8
        MKL_NUM_THREADS : [not set]
ATen parallel backend: OpenMP

>>> numba.get_num_threads()
32

>>> torch.get_num_threads()
32

>>> print(torch.__config__.parallel_info())
ATen/Parallel:
        at::get_num_threads() : 32
        at::get_num_interop_threads() : 128
OpenMP 201511 (a.k.a. OpenMP 4.5)
        omp_get_max_threads() : 32
Intel(R) oneAPI Math Kernel Library Version 2021.1-Product Build 20201104 for Intel(R) 64 architecture applications
        mkl_get_max_threads() : 32
Intel(R) MKL-DNN v2.7.3 (Git Hash N/A)
std::thread::hardware_concurrency() : 256
Environment variables:
        OMP_NUM_THREADS : 8
        MKL_NUM_THREADS : [not set]
ATen parallel backend: OpenMP

>>> numba.set_num_threads(2)
>>> numba.get_num_threads()
2

>>> torch.get_num_threads()
32

>>> torch.__version__
'2.1.0a0+29c30b1'

>>> numba.__version__
'0.57.1+1.gc785c8f1f'

>>> exit()
root@x:/workspace/nemo# exit
exit
```

I have opened an issue on numba https://github.com/numba/numba/issues/9387. 
 Until numba fixes the issue one possible solution is to first read the torch num_threads, and reset them back post numba.set_num_threads().  This isn't specific to hybrid models, I dunno why it shows up only there. In either case, I'll send a PR for temporary fix. 

Basically before I call get or set numba threads, I should cache pytorch threads then set that explicitly after numba thread set ? I have opened a PR, if there are other locations and you prefer to fix by yourself, I can close it. Perfect, let's get yours passing the tests and merged  Adding this here just for completnes; found it while researching something else. Lightning 2.1.0 changelog mentions:
> - If not set by the user, Lightning will set `OMP_NUM_THREADS` to `num_cpus / num_processes` when launching subprocesses (e.g. when DDP is used) to avoid system overload for CPU-intensive tasks ([#18677](https://github.com/Lightning-AI/lightning/pull/18677))
 Hmm actually were seeing some segfaults randomly in NeMo ASR and it might be due to the numba fix PR (I saw the segfault first on that pr but thought it was random fluke). 

We might have to revert that pr for the time being and depend on numba to fix it properly  Hmmm, this is strange. I haven't seen anything like this. During training or inference? 

We use pyxis/enroot, which by default injects `OMP_NUM_THREADS=1` following PyTorch >1.9.0, which does so if the environment variable is not set. Lightning 2.10, will on the other hand set the env (if not previously declared) as mentioned earlier. I have run quick experiments with `OMP_NUM_THREADS=num_cpus / num_processes`, but at least on first impression the difference seems negligible. 

The good thing is that issues with oversubscription (due to numba resetting the value to `min(NUMBA_NUM_THREADS, os.sched_getaffinity(), os.num_cpu())`) start arising only when no cpu binding is used. This holds at least on most Linux platforms, but `os.sched_getaffinity()` is not available on Windows and OsX. There's a comment in torch dataloader about this   https://github.com/pytorch/pytorch/blob/763ddb396df4bc14791fbf9149d46d5713a699df/torch/utils/data/dataloader.py#L501-L512

As Numba maintainers pointed out, one approach might be to ensure `NUMBA_NUM_THREADS` is set to match `OMP_NUM_THREADS`, as this is the value to which the global OMP num_threads (and accordingly also torch num_threads) will be reset to on the first numba call. With pyxis/enroot the injection is simple, but it is not general for all use cases (e.g. docker, ...). 

Yet another approach could be to go the route of Pytorch/Lightning and declare both environment variables (if not already set) during NeMo startup (e.g. in exp_manager).  This issue is stale because it has been open for 30 days with no activity. Remove stale label or comment or this will be closed in 7 days. @titu1994 have you noticed any more segfaults?  No thankfully none in 1.23 branch and main. Seems to be very random results during that one-two week period  This issue is stale because it has been open for 30 days with no activity. Remove stale label or comment or this will be closed in 7 days. This issue was closed because it has been inactive for 7 days since being marked as stale.
Tested on a SLURM 23.11 cluster with Pyxis/Enroot 0.16.1/3.4.1 running NGC containers `nemo:23.03`, `nemo:23.08`, `ea-bignlp/ga-participants/nemofw-training:23.11`, and a custom container built from main branch. 

When running single-node multi-gpu training in exclusive mode or without CPU binding, but with `OMP_NUM_THREADS` set to nCPUs/tasks or 1, the hybrid models (conformer_hybrid, or fastconformer_hybrid) ignore this setting and start too many threads which leads to CPU oversubscription. This results in a  performance drop of ~2x. See also https://pytorch.org/docs/stable/notes/multiprocessing.html#avoid-cpu-oversubscription. 

More specifically this happens both when running as SLURM sbatch 
```bash
#SBATCH --nodes=1
#SBATCH --exclusive
#SBATCH --mem=0
#SBATCH --ntasks-per-node=8
#SBATCH --gpus-per-node=8

srun <train_command>
```

or as docker command
```bash
docker run --rm \
    --net=host --uts=host --ipc=host --security-opt=seccomp=unconfined \
    --ulimit=stack=67108864 --ulimit=memlock=-1 \
    <train_command>
```

In SLURM the workaround is to use cpu binding and explicitly set the CPU count per task (directive `#SBATCH --cpus-per-task`). For docker I haven't found a workaround.

Compared to fastconformer_ctc, fastconformer_transducer, conformer_ctc, the issue is present only in the case of hybrid models (conformer or fastconformer). On a DGX-A100 with 256 vCPU and 8 GPU this can be seen by the following vCPU count (as reported by bash command `nproc` ran as part of the `<train_command>`) and number of main processes' thread counts (as reported by [btop](https://github.com/aristocratos/btop) ran on the host). Note that in all cases python calls `os.cpu_count()` and `multiprocessing.cpu_count()` ran from within the `<train_command>` but prior to actual training return 256, and `torch.__config__.parallel_info()` returns the correct values (as `nproc`). 

conformer_ctc
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS env not set [vCPU=192; nthreads: 153]
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS=24 env set [vCPU=24, nthreads: 49]
- exclusive mode, set cpus-per-tasks on srun, OMP_NUM_THREADS=32 env set [vCPU=32, nthreads: 57] 

conformer_hybrid
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS env not set [vCPU=192; nthreads: 217]
- **exclusive mode, cpus-per-task not set, OMP_NUM_THREADS=24 env set [vCPU=24, nthreads: 217]**
- exclusive mode, set cpus-per-tasks on srun, OMP_NUM_THREADS=32 env set [vCPU=32, nthreads: 57] 

fastconformer_ctc
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS env not set [vCPU=192; nthreads: 153]
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS=24 env set [vCPU=24, nthreads: 49]
- exclusive mode, set cpus-per-tasks on srun, OMP_NUM_THREADS=32 env set [vCPU=32, nthreads: 57] 

fastconformer_transducer
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS env not set [vCPU=192; nthreads: 153]
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS=24 env set [vCPU=24, nthreads: 49]
- exclusive mode, set cpus-per-tasks on srun, OMP_NUM_THREADS=32 env set [vCPU=32, nthreads: 57] 

fastconformer_hybrid
- exclusive mode, cpus-per-task not set, OMP_NUM_THREADS env not set [vCPU=192; nthreads: 217]
- **exclusive mode, cpus-per-task not set, OMP_NUM_THREADS=24 env set [vCPU=24, nthreads: 217]**
- exclusive mode, set cpus-per-tasks on srun, OMP_NUM_THREADS=32 env set [vCPU=32, nthreads: 57] 

Unfortunately I wasn't able to find the exact location where this happens. I may be completely wrong, but one possible cause could be that it is related to decoding, or someplace where `torch.set_num_threads()` gets reset based on the value of `*.cpu_count()`.

