support for gpu queue

[mauriliogenovese]

I wrote a simpler implementation of this old pull request to handle a queue of threads to be executed on GPU.
The user can specify the maximum number of parallel threads with the plugin option n_gpu_procs
The multiprocplugin will raise exception if a node require more threads than allowed in a similar way as classic CPU threads.
Note that in this implementation any GPU node will also allocate a CPU slot (is that necessary? We can change that behavior ).
Moreover the plugin doesn't check that the system actually has a cuda capable GPU (we can add such check if you think we need it)

[codecov]

Codecov Report

Attention: Patch coverage is 69.69697% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 63.45%. Comparing base (a17de8e) to head (a642430).

Files	Patch %	Lines
nipype/pipeline/plugins/multiproc.py	67.74%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3642   +/-   ##
=======================================
  Coverage   63.44%   63.45%           
=======================================
  Files         308      308           
  Lines       40891    40921   +30     
  Branches     5657     5665    +8     
=======================================
+ Hits        25945    25966   +21     
- Misses      13910    13916    +6     
- Partials     1036     1039    +3     

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[effigies]

Just to check my understanding: in this model, a GPU-enabled job gets exclusive access to one full GPU, so the GPU queue is simply the number of available GPUs and the number of GPU-enabled jobs? There's no notion of a job acquiring multiple GPUs or partial GPUs?

From some quick searching, it's at least possible (though I don't know how common) to write programs that utilize multiple GPUs, so I think we should allow nodes to be tagged with multiple GPU threads.

If the CPU usage of a process is negligible, I think it would be reasonable to say:

myproc = pe.Node(ProcessInterface(), n_threads=0, n_gpus=2)

[mauriliogenovese]

In the current implementation the user specifies how many n_gpu_procs the plugin should manage and the plugin will reserve those "slots" based on the node.n_threads property. If you think it's useful we can allow the user to specify different values for "gpu_procs" and "cpu_procs" for each node.
What should be the behaviour if the user does not specify the n_gpus property? n_gpus=n_threads?