Make TensorProto element sizes compile-time (#29644)
### 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)
---------
Co-authored-by: Gopalakrishnan Nallasamy <gopalakrishnan.nallasamy@microsoft.com>
