class_docstr: A global sum pooling layer. Pools a graph by computing the sum of its node
  features.
  
  **Mode**: single, disjoint, mixed, batch.
  
  **Input**
  
  - Node features of shape `([batch], n_nodes, n_node_features)`;
  - Graph IDs of shape `(n_nodes, )` (only in disjoint mode);
  
  **Output**
  
  - Pooled node features of shape `(batch, n_node_features)` (if single mode, shape will
  be `(1, n_node_features)`).
  
  **Arguments**
  
  None.
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code: def __init__(self, **kwargs):
      super().__init__(**kwargs)
      self.pooling_op = tf.math.segment_sum
      self.batch_pooling_op = tf.reduce_sum
  
imports: import tensorflow
  from tensorflow.keras import backend
  from tensorflow.keras import constraints, initializers, regularizers
  from tensorflow.keras.layers import Dense, Layer
  from spektral.layers import ops
label_desc: Use image node_id 1 to create a new GlobalSumPool object from inherited base classes: GlobalPool with example: obj = GlobalSumPool()