[Registry] added sliding window to registry

src/finn/custom_op/registry.py

 # make sure new CustomOp subclasses are imported here so that they get
 # registered and plug in correctly into the infrastructure
+from finn.custom_op.fpgadataflow.convolutioninputgenerator import (
+    ConvolutionInputGenerator
+)
 from finn.custom_op.fpgadataflow.streamingfclayer_batch import StreamingFCLayer_Batch
 from finn.custom_op.fpgadataflow.streamingmaxpool_batch import StreamingMaxPool_Batch
 from finn.custom_op.multithreshold import MultiThreshold
@@ -12,3 +15,4 @@ custom_op["MultiThreshold"] = MultiThreshold
 custom_op["XnorPopcountMatMul"] = XnorPopcountMatMul
 custom_op["StreamingMaxPool_Batch"] = StreamingMaxPool_Batch
 custom_op["StreamingFCLayer_Batch"] = StreamingFCLayer_Batch
+custom_op["ConvolutionInputGenerator"] = ConvolutionInputGenerator