Add a placeholder for MultiImageInput

[bojiang]

Description

Motivation and Context

How Has This Been Tested?

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Documentation
• Test, CI, or build
• None

Components (if applicable)

• BentoService (model packaging, dependency management, handler definition)
• Model Artifact (model serialization, multi-framework support)
• Model Server (mico-batching, logging, metrics, tracing, benchmark, OpenAPI)
• YataiService (model management, deployment automation)
• Documentation

Checklist:

• My code follows the bentoml code style, both ./dev/format.sh and
  ./dev/lint.sh script have passed
  (instructions).
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change requires a change in bentoml/gallery example notebooks
• I have sent a pull request to bentoml/gallery to make that change

[codecov]

Codecov Report

Merging #804 into master will decrease coverage by 0.01%.
The diff coverage is 47.05%.

@@            Coverage Diff             @@
##           master     #804      +/-   ##
==========================================
- Coverage   55.89%   55.87%   -0.02%     
==========================================
  Files         114      115       +1     
  Lines        8400     8417      +17     
==========================================
+ Hits         4695     4703       +8     
- Misses       3705     3714       +9     

Impacted Files	Coverage Δ
bentoml/adapters/multi_image_input.py	43.75% <43.75%> (ø)
bentoml/adapters/__init__.py	100.00% <100.00%> (ø)

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