COMPMID-3510 Fix definition of Softmax "Axis"

* Move helper function dim_index_2_num_dims from header to inl file

Signed-off-by: SiCong Li <sicong.li@arm.com>
Change-Id: I204756997316eae8cc2c9d4ba6a91d5974c75965
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/3325
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
diff --git a/arm_compute/core/Helpers.h b/arm_compute/core/Helpers.h
index 8f1426a..f548b86 100644
--- a/arm_compute/core/Helpers.h
+++ b/arm_compute/core/Helpers.h
@@ -809,10 +809,7 @@
  *
  * @return The number of dimensions in the range [0, @p dim_axis]
  */
-inline size_t dim_index_2_num_dims(int32_t dim_axis, int32_t num_dims)
-{
-    return static_cast<size_t>(wrap_around(dim_axis, num_dims)) + 1;
-}
+inline size_t dim_index_2_num_dims(int32_t dim_axis, int32_t num_dims);
 
 /** Convert negative coordinates to positive in the range [0, num_dims_input]
  *