test: Add backend_api_test to test backend APIs

[yinggeh]

What does the PR do?

Tests modified and new GetElementCount and GetByteSize functions.

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
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Commit Type:

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• test

Related PRs:

triton-inference-server/backend#112
triton-inference-server/onnxruntime_backend#304
triton-inference-server/tensorrt_backend#111
triton-inference-server/pytorch_backend#149
triton-inference-server/python_backend#400
triton-inference-server/openvino_backend#103
triton-inference-server/identity_backend#31

Where should the reviewer start?

Review the changes in backend_common.h and backend_common.cc to prevent overflow.
triton-inference-server/backend#112
https://github.com/triton-inference-server/backend/pull/112/files#diff-661bc0ad90b1e6c5a3d0638c5c29f84d0c369e88d204e0627a3d4e875fdea938
https://github.com/triton-inference-server/backend/pull/112/files#diff-5df7365d8ba293940afcb7a2fe4e551a76f75aa8f5db56c0fc44046ec64636a2

Test plan:

L0_input_validation--base

• CI Pipeline ID:
  27941057

Caveats:

Background

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: #xxx