update vision_transformer.py

ppcls/arch/backbone/model_zoo/vision_transformer.py

@@ -12,6 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# Code was based on https://github.com/rwightman/pytorch-image-models/blob/master/timm/models/vision_transformer.py
+
 from collections.abc import Callable
 
 import numpy as np