refactor: Refactor string input check

[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: #101
tensorflow_backend: triton-inference-server/tensorflow_backend#104
pytorch_backend: triton-inference-server/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.

[yinggeh]

Here slightly different condition checks from original tensorflow.cc and libtorch.cc. Removing (*response != nullptr).

I don't think we want to skip sending this error or skip FillStringTensor if RESPOND_AND_SET_NULL_IF_ERROR is called before SetStringInputTensor in tensorflow.cc and libtorch.cc. I believe it was a mistake because other string format checks before this line do not require if (*response != nullptr) to send error or FillStringTensor. Open to discussions.

// Code from tensorflow.cc and libtorch.cc
if ((*response != nullptr) && (element_idx != request_element_cnt)) {
  RESPOND_AND_SET_NULL_IF_ERROR(
      response, TRITONSERVER_ErrorNew(
                    TRITONSERVER_ERROR_INTERNAL,
                    std::string(
                        "expected " + std::to_string(request_element_cnt) +
                        " strings for inference input '" + name + "', got " +
                        std::to_string(element_idx))
                        .c_str()));
  FillStringTensor(
      tensor, tensor_offset + element_idx, request_element_cnt - element_idx);
}