Add artifact loaded, packed, is_ready state check

[parano]

Description

Before #911, we used a slightly restricted pattern to ensure an artifact can only be accessed or saved after it has been packed or loaded from disk. #911 refactored the interface to make it cleaner to implement although it now missed the enforcement of packed or loaded state check before accessing the model. This PR adds that back by wrapping those methods and setting the internal state when pack or load is invoked.

Motivation and Context

This is part of an effort to provide a more flexible artifacts API that will allow custom model training workflows to integrate better with BentoML's model serving workflow #758

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature and improvements (non-breaking change which adds/improves functionality)
• Bug fix (non-breaking change which fixes an issue)
• Code Refactoring (internal change which is not user facing)
• Documentation
• Test, CI, or build

Component(s) if applicable

• BentoService (service definition, dependency management, API input/output adapters)
• Model Artifact (model serialization, multi-framework support)
• Model Server (mico-batching, dockerisation, logging, OpenAPI, instruments)
• YataiService gRPC server (model registry, cloud deployment automation)
• YataiService web server (nodejs HTTP server and web UI)
• Internal (BentoML's own configuration, logging, utility, exception handling)
• BentoML CLI

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change reduces project test coverage and requires unit tests to be added
• I have added unit tests covering my code change
• My change requires a change to the documentation
• I have updated the documentation accordingly

[pep8speaks]

Hello @parano, Thanks for updating this PR.

There are currently no PEP 8 issues detected in this PR. Cheers! 🍻

Comment last updated at 2020-07-21 09:47:35 UTC

[codecov]

Codecov Report

Merging #921 into master will decrease coverage by 0.07%.
The diff coverage is 44.28%.

@@            Coverage Diff             @@
##           master     #921      +/-   ##
==========================================
- Coverage   62.46%   62.38%   -0.08%     
==========================================
  Files         120      121       +1     
  Lines        7878     8000     +122     
==========================================
+ Hits         4921     4991      +70     
- Misses       2957     3009      +52     

Impacted Files	Coverage Δ
bentoml/marshal/marshal.py	27.97% <0.00%> (-0.60%)	⬇️
bentoml/server/api_server.py	74.83% <ø> (ø)
bentoml/utils/flask_ngrok.py	0.00% <0.00%> (ø)
bentoml/server/__init__.py	44.82% <14.28%> (-5.18%)	⬇️
bentoml/artifact/artifact.py	95.23% <98.11%> (+4.32%)	⬆️
bentoml/cli/bento_service.py	70.86% <100.00%> (+0.19%)	⬆️
bentoml/marshal/dispatcher.py	90.27% <100.00%> (+6.60%)	⬆️
bentoml/artifact/sklearn_model_artifact.py	82.05% <0.00%> (+12.82%)	⬆️
... and 1 more

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