Setting max_gpu_fraction as in Tensorflow backend #108
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This PR permits setting the a max_gpu_fraction for the pytorch backend.
Pytorch allows setting the max gpu fraction through the
CUDACachingAllocator. The user of the pytorch_backend can set the memory fraction in the same fashion as in the tensorflow backend. The memory fraction applies to all models.I am a bit uncertain about how to handle the case with multiple GPUs and would appreciate feedback about desired behavior in this case.