feat: modify aten_bilinear from einsum to matmul (#2746)

#### What this does
- Replaces Bilinear Transformation's einsum implementation with matmul

#### Why
- Performance increase from MatMul over Einsum

#### Testing
- Existing tests pass for function implementation

Resolves #2573

Author: fw7th

onnxscript/function_libs/torch_lib/ops/core.py

@@ -1208,12 +1208,34 @@ def aten_bilinear(
     # bias shape: (out_features) - optional
     # output shape: (..., out_features)
 
-    # Use Einsum to compute the bilinear transformation
-    # "...i,oij,...j->...o" means:
-    # - input1[..., i] * weight[o, i, j] * input2[..., j] -> output[..., o]
-    result = op.Einsum(input1, weight, input2, equation="...i,oij,...j->...o")
+    # input1 and input2 must have identical batch dimensions
+    # Use MatMul to compute the bilinear transformation
+    batch_size = op.Shape(input1, start=0, end=-1)
+    input1_shape = op.Shape(input1, start=-1)
+    input2_shape = op.Shape(input2, start=-1)
+    output_shape = op.Shape(weight, start=0, end=1)
+    neg_1 = op.Constant(value_ints=[-1])
+
+    # (out_features, in1_features, in2_features) -> (in1_features, out_features, in2_features)
+    W_permute = op.Transpose(weight, perm=[1, 0, 2])
+
+    # (in1_features, out_features, in2_features) -> (in1_features, out_features * in2_features)
+    W_flat = op.Reshape(
+        W_permute,
+        op.Concat(input1_shape, op.Mul(output_shape, input2_shape), axis=0),
+    )
+
+    # (..., in1_features) @ (in1_features, out_features * in2_features) -> (..., out_features * in2_features)
+    tmp = op.MatMul(input1, W_flat)
+
+    # (..., out_features * in2_features) -> (..., out_features, in2_features)
+    tmp = op.Reshape(tmp, op.Concat(batch_size, output_shape, input2_shape, axis=0))
+
+    # (..., in2_features) -> (..., in2_features, 1)
+    #   -> (..., out_features, in2_features) @ (..., in2_features, 1)
+    #   -> (..., out_features, 1) -> (..., out_features)
+    result = op.Squeeze(op.MatMul(tmp, op.Unsqueeze(input2, neg_1)), neg_1)
 
-    # Add bias if provided
     if bias is not None:
         result = op.Add(result, bias)