Reduce sub process memory

[ErraticO]

What this PR does / why we need it: Due to the independent memory space between processes, loading a model in a child process will result in loading multiple times. In the old way, if there were three child processes, the model would be loaded three times and the memory would increase by nearly three times. In the new way, load the model once in the parent process and then share it with the child process through Copy-On-Write (COW), with almost no increase in memory.

Which issue(s) this PR fixes:

Fixes # Fix memory increase caused by loading models in child processes.

Special notes for your reviewer:

[seldondev]

Hi @ErraticO. Thanks for your PR.

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