Support multiple providers init tasks

[avive]

Add support for using more than 1 provider for creating a post init job.

The motivation for this are users with multiple gpus on their systems. For example, a laptop with 2 gpus or a desktop with 2 discrete gpu cards and a supported gpu on the motherboard. We'd like to enable these users to create a post init in less time and fully utilize their system gpu compute resources.

Add API method where user can provide an array of distinct system providers for creating a post job. The implementation should utilize all providers in parallel to create the post init data. Note that the gpu-post lib already supports executions on multiple providers in parallel.

@moshababo