Subject: Extend the tensor element types to the float6 formats added by ONNX 1.23

ONNX 1.23 appends FLOAT6E2M3 and FLOAT6E3M2 to TensorProto::DataType. Two
consteval self-checks tie onnxruntime's own tables to the size of that enum
and both stop resolving as soon as it grows:

  - kTensorProtoDataTypeElementSizes is declared with the enum's ARRAYSIZE but
    initialised only through INT2, so the new slots default to zero and the
    completeness check reads them as non-numeric types.
  - IsTensorProtoToOrtElementTypeMapBijective compares that ARRAYSIZE against
    the public ONNXTensorElementDataType enum, which still ends at FLOAT8E8M0.

Append both formats to the public enum, which keeps every existing value in
place, and move the bijection's upper bound to the new last entry. The
mapping switch needs no new case: ONNX and onnxruntime agree on 27 and 28, so
the existing pass-through default covers them.

Record both as byte-granular in the element-size table, matching every packed
sub-byte type already listed. That table feeds only the big-endian
byte-swapping helper, where byte granularity is exactly what the UINT4, INT4,
FLOAT4E2M1, UINT2 and INT2 entries already express.

The wider initialiser does not fit the narrower ONNX 1.22 enum, so this
requires ONNX 1.23. verified 2026-09-26

--- a/include/onnxruntime/core/session/onnxruntime_c_api.h
+++ b/include/onnxruntime/core/session/onnxruntime_c_api.h
@@ -223,6 +223,9 @@
   ONNX_TENSOR_ELEMENT_DATA_TYPE_INT2,   // maps to 4 packed int2 values (size == 1 byte)
   // Float8E8M0 type introduced in ONNX 1.21. 8-bit float with 8 exponent bits, 0 mantissa bits, no sign bit.
   ONNX_TENSOR_ELEMENT_DATA_TYPE_FLOAT8E8M0,  // Non-IEEE floating-point format, all values are powers of two
+  // Float6 types were introduced in ONNX 1.23.
+  ONNX_TENSOR_ELEMENT_DATA_TYPE_FLOAT6E2M3,  // Non-IEEE 6-bit floating-point format, stored packed
+  ONNX_TENSOR_ELEMENT_DATA_TYPE_FLOAT6E3M2,  // Non-IEEE 6-bit floating-point format, stored packed
 } ONNXTensorElementDataType;
 
 // Synced with onnx TypeProto oneof
--- a/onnxruntime/core/framework/onnxruntime_map_type_info.h
+++ b/onnxruntime/core/framework/onnxruntime_map_type_info.h
@@ -26,8 +26,9 @@
     return ONNX_TENSOR_ELEMENT_DATA_TYPE_UNDEFINED;
   }
 
-  // The enums have the same values through FLOAT4E2M1. Their final three entries
-  // differ because ONNX inserted FLOAT8E8M0 before UINT2 and INT2.
+  // The enums have the same values through FLOAT4E2M1, and again from FLOAT6E2M3
+  // on. The three entries between differ because ONNX inserted FLOAT8E8M0 before
+  // UINT2 and INT2.
   switch (data_type) {
     case ONNX_NAMESPACE::TensorProto_DataType_FLOAT8E8M0:
       return ONNX_TENSOR_ELEMENT_DATA_TYPE_FLOAT8E8M0;
@@ -42,7 +43,7 @@
 
 consteval bool IsTensorProtoToOrtElementTypeMapBijective() {
   constexpr size_t ort_element_type_count =
-      static_cast<size_t>(ONNX_TENSOR_ELEMENT_DATA_TYPE_FLOAT8E8M0) + 1;
+      static_cast<size_t>(ONNX_TENSOR_ELEMENT_DATA_TYPE_FLOAT6E3M2) + 1;
   constexpr size_t onnx_element_type_count = ONNX_NAMESPACE::TensorProto_DataType_DataType_ARRAYSIZE;
   if constexpr (onnx_element_type_count != ort_element_type_count) {
     return false;
--- a/onnxruntime/core/framework/tensorprotoutils.h
+++ b/onnxruntime/core/framework/tensorprotoutils.h
@@ -58,6 +58,8 @@
         sizeof(uint8_t),   // FLOAT8E8M0
         sizeof(uint8_t),   // UINT2
         sizeof(uint8_t),   // INT2
+        sizeof(uint8_t),   // FLOAT6E2M3
+        sizeof(uint8_t),   // FLOAT6E3M2
     };
 
 consteval bool IsTensorProtoDataTypeElementSizeMapComplete() {