Porting to TF2

Fixes #75 (closed)

bob/learn/tensorflow/loss/center_loss.py

-import tensorflow as tf
-
-
-# TODO(amir): replace parent class with tf.Module in tensorflow 1.14 and above.
-# * pass ``name`` to parent class
-# * replace get_variable with tf.Variable
-# * replace variable_scope with name_scope
-class CenterLoss:
-    """Center loss."""
-
-    def __init__(self, n_classes, n_features, alpha=0.9, name="center_loss", **kwargs):
-        super().__init__(**kwargs)
-        self.n_classes = n_classes
-        self.n_features = n_features
-        self.alpha = alpha
-        self.name = name
-        with tf.compat.v1.variable_scope(self.name):
-            self.centers = tf.compat.v1.get_variable(
-                "centers",
-                [n_classes, n_features],
-                dtype=tf.float32,
-                initializer=tf.compat.v1.constant_initializer(0.0),
-                trainable=False,
-            )
-
-    def __call__(self, sparse_labels, prelogits):
-        with tf.compat.v1.name_scope(self.name):
-            centers_batch = tf.gather(self.centers, sparse_labels)
-            diff = (1 - self.alpha) * (centers_batch - prelogits)
-            self.centers_update_op = tf.compat.v1.scatter_sub(
-                self.centers, sparse_labels, diff
-            )
-            center_loss = tf.reduce_mean(
-                input_tensor=tf.square(prelogits - centers_batch)
-            )
-        tf.compat.v1.summary.scalar("loss_center", center_loss)
-        # Add histogram for all centers
-        for i in range(self.n_classes):
-            tf.compat.v1.summary.histogram(f"center_{i}", self.centers[i])
-        return center_loss
-
-    @property
-    def update_ops(self):
-        return [self.centers_update_op]