Add relu6 supports.

PiperOrigin-RevId: 447585136

Author: Xhark

tensorflow_model_optimization/python/core/quantization/keras/default_8bit/default_8bit_quantize_registry.py

@@ -502,7 +502,7 @@ def get_output_quantizers(self, layer):
                        'Default8BitActivationQuantizeConfig.'.format(
                            layer.activation))
 
-    if layer.activation.__name__ in ['relu', 'swish', 'gelu']:
+    if layer.activation.__name__ in ['relu', 'swish', 'gelu', 'relu6']:
       # 'relu' should generally get fused into the previous layer.
       return [quantizers.MovingAverageQuantizer(
           num_bits=8, per_axis=False, symmetric=False, narrow_range=False)]