refactor: Add string input checks

[yinggeh]

What does the PR do?

Move each individual check of string input in model backend repos to the bakend repo.

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• refactor

Related PRs:

backend: triton-inference-server/backend#101
tensorflow_backend: triton-inference-server/tensorflow_backend#104
pytorch_backend: #136
onnxruntime_backend: triton-inference-server/onnxruntime_backend#263
python_backend: triton-inference-server/python_backend#370

Test plan:

• CI Pipeline ID:
  17021222 and 17057045

Background

Initially just to add missing string input tensor checks to python_backend. Refactored because duplicate code from other model backends were found.

[Tabrizian]

Nice refactoring.