multiprocessing start_method setup runtime exception

[DrizzlingCattus]

Describe the bug

The problem occurs when I try to build the bentoml service as below.

# do normal bento pack & save
python build_bentoservice.py

The problem occurs when using the bentoml master branch, and there is no problem in version 0.11.0.

To Reproduce

Expected behavior

Screenshots/Logs

Traceback (most recent call last):
  File "/app/BentoML/bentoml/__init__.py", line 40, in <module>
    multiprocessing.set_start_method('fork')
  File "/usr/lib/python3.8/multiprocessing/context.py", line 243, in set_start_method
    raise RuntimeError('context has already been set')
RuntimeError: context has already been set

Environment:

• OS

NAME="Ubuntu"
VERSION="18.04.3 LTS (Bionic Beaver)"

• Python Version : 3.8.3
• BentoML Version : master branch version (lastest commit aa120e4)

Additional context

[yubozhao]

Thanks for find the bug and create a PR at #1461.

Let me know when the PR is ready and we will go from there.

[bojiang]

FYI, in this PR #1466, set_start_method was moved back to the API server.
@DrizzlingCattus

[DrizzlingCattus]

@bojiang ok thank you!

[bojiang]

The final solution #1476