[WebNN] Remove constraints for Gemm's C input (microsoft#26273)

Now WebNN implementation for gemm's C operand has supported
unidirectional broadcasting, which is align with ONNX spec. Removing
constraints for Gemm's C input as which should be covered in ORT kernel.

Author: Honry

js/web/docs/webnn-operators.md

@@ -46,7 +46,7 @@ platforms. Check the [WebNN status](https://webmachinelearning.github.io/webnn-s
 | GatherElements | ai.onnx(11-12, 13+) | gatherElements | |
 | GatherND | ai.onnx(11, 12, 13+) | gatherND | Only supports 'batch_dims' == 0 |
 | Gelu | ai.onnx(20+) | gelu | |
-| Gemm | ai.onnx(7-8, 9-10, 11-12, 13+) | gemm | Only supports 1-D 'C' input |
+| Gemm | ai.onnx(7-8, 9-10, 11-12, 13+) | gemm | |
 | GlobalAveragePool | ai.onnx(7+) | averagePool2d | Only supports 4-D input |
 | GlobalMaxPool | ai.onnx(7+) | maxPool2d | Only supports 4-D input |
 | GlobalLpPool| ai.onnx(7+) | l2Pool2d | Only supports 4-D input, 'p' value is 2 |

onnxruntime/core/providers/webnn/builders/impl/gemm_op_builder.cc

@@ -250,29 +250,6 @@ bool GemmOpBuilder::IsOpSupportedImpl(const GraphViewer&,
       std::vector<int64_t> c_shape;
       if (!GetShape(*input_defs[c_idx], c_shape, logger))
         return false;
-
-      size_t c_dim = c_shape.size();
-
-      if (c_dim > 1) {
-        // TODO: Supports other shape of C.
-        // Currently WebNN implementation in Chromium only supports 1-D C.
-        return false;
-      }
-      if (c_dim == 0) {
-        LOGS(logger, VERBOSE) << "C of Gemm is a scalar";
-      } else {
-        auto c_size = c_shape[c_dim - 1];
-        NodeAttrHelper helper(node);
-        const auto transB = helper.Get("transB", 0);
-        if (c_size != (transB == 0 ? b_shape[1] : b_shape[0])) {
-          LOGS(logger, VERBOSE) << "C of Gemm must be a vector of b_shape["
-                                << (transB == 0 ? "1" : "0") << "]"
-                                << " b_shape: [" << b_shape[0] << ", " << b_shape[1] << "]"
-                                << " c_size: " << c_size;
-
-          return false;
-        }
-      }
     }
   }