COMPMID-3656: Disabled reduce_axis in LOG_SOFTMAX and SOFTMAX

Our implementation of reduce_axis is only compliant for default_axis.

Validate will throw an error when trying to use a different axis.

Change-Id: I4c02aa055bb4474593a3114ec9c83884d3c9120f
Signed-off-by: morgolock <pablo.tello@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/3658
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: SiCong Li <sicong.li@arm.com>
diff --git a/docs/00_introduction.dox b/docs/00_introduction.dox
index ed04bc8..906ddf2 100644
--- a/docs/00_introduction.dox
+++ b/docs/00_introduction.dox
@@ -260,9 +260,10 @@
  - Deprecated functions / interfaces:
    - Non-descriptor based interfaces for @ref NEThreshold, @ref CLThreshold
    - In @ref NESoftmaxLayer, @ref NELogSoftmaxLayer, @ref CLSoftmaxLayer, @ref CLLogSoftmaxLayer and @ref GCSoftmaxLayer :
-      "axis" has been renamed to "reduce_end_axis", which is the last axis (inclusive) before which all dimensions are reduced/collapsed.
-      The default "axis" (now "reduce_end_axis") value for @ref NESoftmaxLayer and @ref NELogSoftmaxLayer is changed from -1 to 0.
-      The default "axis" (now "reduce_end_axis") value for @ref CLSoftmaxLayer, @ref CLLogSoftmaxLayer and @ref GCSoftmaxLayer is changed from 1 to 0.
+      The default "axis" value for @ref CLSoftmaxLayer, @ref CLLogSoftmaxLayer and @ref GCSoftmaxLayer is changed from 1 to 0.
+      Only axis 0 is supported.
+      The default "axis" value for @ref NESoftmaxLayer, @ref NELogSoftmaxLayer is changed from 1 to 0.
+      Only axis 0 is supported. 
  - The support for quantized data types has been removed from @ref CLLogSoftmaxLayer due to implementation complexity.
 
 v20.05 Public major release