[HLSCustomOp] update expected cycles for StreamingMaxPool_Batch

based on loop nest

src/finn/custom_op/fpgadataflow/streamingmaxpool_batch.py

@@ -96,8 +96,10 @@ class StreamingMaxPool_Batch(HLSCustomOp):
         return np.prod(folded_oshape[:-1])
 
     def get_exp_cycles(self):
-        # Channels * batch size * odim * odim
-        return np.prod(self.get_folded_output_shape()[:-1])
+        # derived from StreamingMaxPool_Batch loop nest
+        k = self.get_nodeattr("PoolDim")
+        ifm_dim = self.get_nodeattr("ImgDim")
+        return ifm_dim * (ifm_dim + (ifm_dim / k))
 
     def get_instream_width(self):
         dt_bits = self.get_input_datatype().bitwidth()