Make TensorProto element sizes compile-time

[GopalakrishnanN]

Description

• Replace the function-local unordered_map used by GetElementSizeOfTensor with a directly indexed constexpr table.
• Validate at compile time that every numeric TensorProto_DataType has a nonzero entry while UNDEFINED and STRING remain unsupported.
• Make GetElementSizeOfTensor usable in constant expressions and safely return zero for out-of-range enum values.
• Add an independent exhaustive static_assert test covering every current ONNX tensor data type.

Motivation and Context

The previous map performed dynamic first-use initialization and heap-backed hash lookup for a fixed enum-to-size relation. A constexpr table moves completeness checking to compilation, removes runtime initialization, and reduces the generated framework object and library size.

Local Windows RelWithDebInfo measurements, using a stabilized build and forcing only tensorprotoutils.cc stale:

Metric	Before	After	Change
Incremental build	14.01 s	12.04 s	-14.1%
tensorprotoutils.obj	6,500,915 B	6,217,508 B	-283,407 B (-4.36%)
onnxruntime_framework.lib	196,933,848 B	196,570,904 B	-362,944 B (-0.18%)

Validation

• lintrunner -a onnxruntime/core/framework/tensorprotoutils.h onnxruntime/core/framework/tensorprotoutils.cc onnxruntime/test/framework/tensorutils_test.cc
• cmake --build build/Windows/RelWithDebInfo --config RelWithDebInfo --target onnxruntime_test_all --parallel 8
• onnxruntime_test_all.exe --gtest_filter="TensorProtoUtilsTest.*" (10/10 passed)

[Copilot]

Pull request overview

This PR refactors utils::GetElementSizeOfTensor from a runtime-initialized unordered_map lookup to a constexpr, directly-indexed table in tensorprotoutils.h, enabling constant-expression use and compile-time completeness validation for ONNX TensorProto_DataType element-size mappings.

Changes:

• Replaces the function-local unordered_map with an inline constexpr std::array lookup table.
• Makes GetElementSizeOfTensor constexpr noexcept, returning 0 for out-of-range enum values.
• Adds an exhaustive compile-time (static_assert) test ensuring the mapping is constexpr and covers all current ONNX tensor data types.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
onnxruntime/core/framework/tensorprotoutils.h	Adds constexpr element-size table + compile-time completeness checking; inlines GetElementSizeOfTensor as constexpr.
onnxruntime/core/framework/tensorprotoutils.cc	Removes the old unordered_map-based implementation of GetElementSizeOfTensor.
onnxruntime/test/framework/tensorutils_test.cc	Adds an exhaustive constexpr static_assert test validating element sizes for all ONNX tensor data types and out-of-range behavior.