add ggml_add_cast API function

this function works like ggml_add, but accepts a data type for the resulting tensor.
only supported for quantized src0 input.

ggml.c

@@ -5115,6 +5115,44 @@ struct ggml_tensor * ggml_add_inplace(
     return ggml_add_impl(ctx, a, b, true);
 }
 
+// ggml_add_cast
+
+static struct ggml_tensor * ggml_add_cast_impl(
+        struct ggml_context * ctx,
+        struct ggml_tensor * a,
+        struct ggml_tensor * b,
+        enum   ggml_type     type) {
+    // TODO: support less-strict constraint
+    //       GGML_ASSERT(ggml_can_repeat(b, a));
+    GGML_ASSERT(ggml_can_repeat_rows(b, a));
+    GGML_ASSERT(ggml_is_quantized(a->type)); // currently only supported for quantized input
+
+    bool is_node = false;
+
+    if (a->grad || b->grad) {
+        // TODO: support backward pass for broadcasting
+        GGML_ASSERT(ggml_are_same_shape(a, b));
+        is_node = true;
+    }
+
+    struct ggml_tensor * result = ggml_new_tensor(ctx, type, a->n_dims, a->ne);
+
+    result->op   = GGML_OP_ADD;
+    result->grad = is_node ? ggml_new_tensor(ctx, GGML_TYPE_F32, a->n_dims, a->ne) : NULL;
+    result->src[0] = a;
+    result->src[1] = b;
+
+    return result;
+}
+
+struct ggml_tensor * ggml_add_cast(
+        struct ggml_context * ctx,
+        struct ggml_tensor * a,
+        struct ggml_tensor * b,
+        enum   ggml_type     type) {
+    return ggml_add_impl(ctx, a, b, false);
+}
+
 // ggml_add1
 
 static struct ggml_tensor * ggml_add1_impl(
@@ -8317,8 +8355,9 @@ static void ggml_compute_forward_add_q_f32(
     const int nth = params->nth;
 
     const enum ggml_type type = src0->type;
+    const enum ggml_type dtype = dst->type;
     ggml_to_float_t const dequantize_row_q = type_traits[type].to_float;
-    ggml_from_float_t const quantize_row_q = type_traits[type].from_float;
+    ggml_from_float_t const quantize_row_q = type_traits[dtype].from_float;
 
     // we don't support permuted src0 or src1
     GGML_ASSERT(nb00 == GGML_TYPE_SIZE[type]);
@@ -8368,7 +8407,11 @@ static void ggml_compute_forward_add_q_f32(
         // add src1
         ggml_vec_acc_f32(ne00, wdata, src1_row);
         // quantize row to dst
+        if (quantize_row_q != NULL) {
             quantize_row_q(wdata, dst_row, ne00);
+        } else {
+            memcpy(dst_row, wdata, ne0*nb0);
+        }
     }
 }
 

ggml.h

@@ -670,6 +670,12 @@ extern "C" {
             struct ggml_tensor  * a,
             struct ggml_tensor  * b);
 
+    GGML_API struct ggml_tensor * ggml_add_cast(
+            struct ggml_context * ctx,
+            struct ggml_tensor  * a,
+            struct ggml_tensor  * b,
+            enum   ggml_type      type);
+
     GGML_API struct ggml_tensor * ggml_add1(
             struct ggml_context * ctx,
             struct ggml_tensor  * a,