Explicitly fall back on floatx() in case compute_dtype is None.

PiperOrigin-RevId: 427458260
Change-Id: I997f6bf9a3c4f466e3a952b0c29320c15a918abb

Author: Johannes Ballé

tensorflow_compression/python/entropy_models/continuous_base.py

@@ -80,6 +80,8 @@ def __init__(self,
     self._tail_mass = float(tail_mass)
     if bottleneck_dtype is None:
       bottleneck_dtype = tf.keras.mixed_precision.global_policy().compute_dtype
+    if bottleneck_dtype is None:
+      bottleneck_dtype = tf.keras.backend.floatx()
     self._bottleneck_dtype = tf.as_dtype(bottleneck_dtype)
     self._laplace_tail_mass = float(laplace_tail_mass)