make pre_nms_limit configurable

mrcnn/model.py

@@ -284,7 +284,7 @@ class ProposalLayer(KE.Layer):
 
         # Improve performance by trimming to top anchors by score
         # and doing the rest on the smaller subset.
-        pre_nms_limit = tf.minimum(6000, tf.shape(anchors)[1])
+        pre_nms_limit = tf.minimum(self.config.PRE_NMS_LIMIT, tf.shape(anchors)[1])
         ix = tf.nn.top_k(scores, pre_nms_limit, sorted=True,
                          name="top_anchors").indices
         scores = utils.batch_slice([scores, ix], lambda x, y: tf.gather(x, y),